Regardless of your daughters sexual preference, you really need to take a hard look at yourself OP, because you have a lot to learn about gay (and other not straight) people. Why is it that the idea of your DD being gay is so horrible to you? Why would you hope she is not gay? if she is, that's like wishing for a different kid. If you are truly worried about her happiness, then acccepting her for who she is is the best thing you can do for her. You say you are worried about her life being harder, but if she is gay, your negative feelings about it are probably the thing making her life the most difficult right now.
I am gay. Let me tell you what my life is like: It's great! I met my wife when I was 19, and we have been together ever since. We have a wonderful daughter and are trying for another baby. Sometimes I don't get enough sleep when my toddler wakes me up at night, sometimes we worry about money, last week we all got food poisoning at the same time, and let me tell you, that was hard! But I'm pretty sure that can happen to straight people, too, lol. Every once in a while I'll encounter a little bit of discrimination, a few weeks ago a woman at play group asked which one of us was the "real mom", for example, but that makes me think that I just need to keep working to make the word a better place, it doesn't make me feel like my family shouldn't exist! We have gone through some very hard times, including the loss of a baby, but not because we are gay, because we are human, and sometimes life is hard for everyone. We got through the tough times because we love and support each other.
After you have seriously examined your own attitude (maybe PFLAG can help you out), you can just say, "Hey, sorry if I'm being nosy, but are you and ____ dating? You know I'll love and support you no matter what".