Thank you, Cynthia, for setting some ground rules.
That being said, I will go ahead and post my thoughts.
I am a single mother by choice, and a lesbian, due in July with my much anticipated first child, conceived via donor insemination. I could not be more excited and utterly delighted!
I can see you are operating from some assumptions that I would say, based on my personal experience, are false assumptions. I know many people who operate from your world view (as I perceive it) and get these questions occassionally, so I'd be honored if you'd listen to my side of the story.
The first false assumption in your post, based on my experience and that of my friends/acquaintences, is that everyone is heterosexual by biology and choice. That is not true. There is a wide variety of orientations that fall along a continuum, both in human nature and the animal kingdom. Same sex orientation and affectual connection are a natural part of the continuum, though an admittedly small percentage. The percentages range between 1 and 10% of the population, depending on which study you look at. But almost all studies/scientists/psychologists (and queer people, lol) etc agree that there DOES exist a natural population of same sex oriented people, both male and female. This is my belief.
The second false assumption in your post, is that there is a "gay lifestyle", as if we all lived and acted the same both sexually and just in life in general. Unlike religious groups, or races, or tribes or socio-economic groups etc, which share either a racial make up or common belief system or social status, the ONLY thing gay & lesbian folks have in common is their orientation (the fact that we fall in love with people of the same gender). That's IT. We have no common religion, no common race, no common socio-economic group, no common cultural shared group beliefs. There is such a huge variety! By saying "your gay lifestyle" - it's just limiting and assumes we all party and screw around and are only interrested in having sex with whomever we can as often as we can. That's incredibly insulting and, well, just plain ignorant of the complexities of real life, and reduces me to a characture. There is no gay lifestyle. I have a real life. I am a real person. I am not reducible to a stereotype. I know why there IS a stereotype - ie gay men are party boys and lesbians are macho dykes who wear lots of plaid shirts - is because 1) some gay men ARE party boys and some lesbians do wear plaid, lol and 2) reducing real people to stereotypes makes others feel safe.
The third false assumption in your post is that I CHOOSE not to be in a heterosexual relationship with a man. Even if I DID get together with a man, it still would never be a hetero relationship, because my nature is not hetero! I am biologically female and biologically lesbian. That is the way my brain is wired. I am not interrested in men. I find nothing attractive about them, though I love many of them as friends and family members. I AM attracted to women, the same way you are attracted to the opposite sex. I find my emotional fulfillment in women who are wired biologically just like me (and not all of them, to be sure, lol, I'm picky). I do not choose not to be hetero. I AM not. I choose to be who I really am. I choose to live an honest life, not living a lie pretending to be something I'm not.
As to the "fruit of a heterosexual relationship" - hmmm. What an odd way to put it. I believe your assumption here is that only a man and woman, married and committed to one another, are capable and elected to raise biological children, and that families should be limited to only those families that "qualify" under your very limited definition of a nuclear family. In my point of view, a family is a much broader term. It encompasses extended family, "chosen" family , god-parents, guardians, step-families, multi-generational families, adoptive families, etc. My term for family encompasses much more than ONLY the nuclear family. Some children are lucky that they have nice nuclear families with a mom, dad and kids. That does not mean children who are being raised in other family systems are losing out! Just because your biological "private parts" fit together easily does not mean you are a good parent or potential parent.
Parenting means much more than the ability to have sex and produce a biological offspring! In gay & lesbian families, there are no "oops" babies (except in gay-straight couples where one "comes out" after being married & a parent). Every child is a much wanted and planned for child. No one "accidently" becomes a parent. For lesbians, the path to parenthood is biologically easier, since it usually only involves a source of sperm. For gay men, the path is more difficult and always involves outside sources - like an adoption agency, social workers, surrogate agencies & counselors, foster-care social workers etc. With all the "hoops" gay men have to jump through to have kids, you can bet that 1) only the serious dads will follow through and 2) those kids are very, very wanted & planned for and well taken care of. Just because a man & woman have the capability to have sex and produce children does not mean in any way that those people are going to be good parents. Likewise, just because two men or two women do not have "complimentary" private parts does not disqualify them from being good parents. A good parent is full of love, compassion and knowledge, regardless of gender or sexual orientation.
Regarding "parenting being an unnatural act" for a gay or lesbian person. Why does who you fall in love with matter in parenting? A parent is a parent, not a partner or spouse. The roles are completely different. Why would nurturing and caring for a child be affected by your orientation at all? I can see the concern/assumption is that queer people, usually implied to be gay men, are pedophiles. Sorry, but most pedophiles are straight married men who molest their own children. Statistically, the most dangerous person to a child is his/her own biological father. Parenting is a very natural act for those with the temperment, regardless of orientation or gender. I know wonderful gay & lesbian parents and absolutely crappy straight parents. A more useful way of determining if a parent is a good one is HOW they are with kids, rather than WHO they are and who they fall in love with. A natural act is to nurture if you are a nurturing person, regardless of orientation. A natural parent (noun), one with the personality to undertake such a mission, will naturally want to parent (verb), regardless of their orientation.
Also, regarding "natural" parenting - I'm assuming YOU assume "natural" conception to be defined as intercourse only, and all other variations (donor insemination, IVF etc) are "unnatural" because they are not intercourse. Well, it all basically comes down to an egg and a sperm in the end, doesn't it? Regardless of HOW what get's where. Doesn't get much more natural than that. I am personally exceedingly grateful to my donor for helping me to conceive my child!
I hope this helps you understand a different world veiw, a different paradigm.