I think it is not hard to draw the line, but of course it is different for different people. Personally, we draw the line using our sense of how aware the child may be. An infant's awareness is different than a toddler's and we can draw our lines just fine. There are no moral problems here for us, but we don't do anything we consider questionable and I do not believe our kids have ever seen anything that made us feel they had seen too much or anything that upset them. The other line to draw is whether we can relax and enjoy ourselves.
We have always had our ways of being sneaky when we feel the need, by being quiet and under covers or slipping away to an odd part of the house while they are doing their own thing during daytime hours. We do put ourselves in a position of taking a risk of being walked in on, but we can hear footsteps well so we can cover up and by the time the door opens, we're just cuddling and not at all "graphic." That's okay. I am not shamed or anything if that cuddling suggests something more yet we are not on display either. My kids are quite a bit older now, so they can get the idea of a closed door and they are capable of being alone doing other things in the house. We no longer cosleep, but we have coslept with up to three kids. (We had a whole room floor covered with futons at that time.)
I don't see any problem with doing it while nursing an infant who is truly not aware. I would consider an infant in the la-la land of sleep-nursing not disturbed by a gentle quickie from behind, even though it's not very "fun". But it really is not "fun" so why go for it, generally? I can darn well tell the difference between a sleeping unaware infant (who simply doesn't judge the activities of others) and a near-toddler who is quite curious about what the adults are up to. Later, a sleeping child with enough room to be undisturbed, no problem. You can probably tell if they are likely to be disturbed just based on your awareness of their environment and sleep patterns. A bouncy queen size bed of course is entirely different from multiple mats on a non-bouncy floor. Full darkness by itself can provide a buffer of privacy as well. So some things depend on the setup as well. Blankets and carefulness with noise are worth it if it is the only way to be intimate.
If you are like most cosleeping families in our culture, you are choosing to not set up an individual bedroom for your child even though you have one available. Well, that room should have an extra bed in it anyway IMO whether you call it a guest bed or whatever. You don't have to send your child to a separate room at night to be able to have one available for you. We generally always had more beds in the house that might be used for nursing and napping anyway because we had multiple young children and sometimes just needed extra spaces for different people's needs. We actually needed an extra place for an adult just to get a nap alone sometimes, as my dh sometimes had to work odd very late and very early hours and had to catch up on sleep at a different time from the children's sleep. We also used the couch more for DTD during that time of our lives after kids were all asleep and of course we did have a lot less sex regardless. That was mainly because we were unpredictably on call and tired though. Interruption for basic baby needs was always possible when they were little, no matter where we went. Location was the easy part IME.
I think cultural norms are affected also by the relative luxury of privacy we have. Americans often have more rooms available than other families in other countries. When a family shares a single room or tiny house, as has been and is the case in many times and places, they devise different methods and standards for how discreet they are. If you share a room by necessity, a child may wake up in the night and notice adults being "wrestly" and noisy in their shared blankets. How shocking is that? I don't find it shocking. However, I do have the luxury of more privacy than that and I do use it. It's just more relaxing and comfortable.
We don't have any extra rooms, so some of this doesn't apply to us, but I love this post. Mind you, I think the issue of privacy being a luxury is multi-faceted. It's a luxury, but it's also a cultural expectation, to some extent. In many times and places, that level of luxury wasn't an expectation, so I'm sure people handled it differently. (I'm extremely edgy about being heard or seen or anything...but I doubt I would be if I'd grown up in a place where sex and sexuality was more public, yk?)