I think we need a little more information before giving an answer. First of all, how old is your daughter? Children that are very young don’t really need a lot of detailed info on sexual matters. For example, if she’s 5, you just say this is something a man & a woman do together that feels good, but it’s not for you at your age. If she’s older, say early teens, she prbly has some idea what was going on. In that case you say I’m sorry you saw that, I’ll be more careful to ensurevprivacy next time, as this is smthg adults do but only in complete privacy.
Also, I think you should tell us what the circumstances were - I assume you were at home, right? Were you in the bedroom when this happened, or somewhere like the living room? If you were not in a more ‘private’ setting, like the bedroom, you should apologize & tell her we should not have been somewhere she could possibly see this. If you were in the bedroom & the door was closed (but not locked) & she barged in, tell her you need privacy & that she should knock before entering, and also that next time, you’ll lock the door.
Anyway, please provide more details, such as your daughter’s age, when & where this happened, etc., in order to give the best answer to your question.