yes the sex scenes have got to be the hardest. or of the hardest. it's crazy. it's not just if your SO is there, cuz there are TONS of crew ppl that will be there. sometimes in certain cases, if the actor(s) are really really uncomfy, they might try to exit all the ppl that do not absolutely need to be there, to lessen the nerves of the actors but of course that can't always happen.
yes, sometimes actors' partners are present when the actor is filming a sex scene, it does happen. after all there have been many films made over the decades lol. does it happen often? probably not. most ppl can't take watching that kind of thing, even if it's just a kiss or something small. and then there are some ppl who want to watch to make sure their partner doesn't try to pull anything lmao. but yes it does happen occasionally.
I don't think I could be able to do a sex scene, or at least not for a long time. I can barely stand to kiss another actor, let alone do a sex scene. it's something I have to learn to get over.