As someone who has heard her parents going at it (both as a teenager and an adult) I promise she'll get over it and be completely fine. Sex is healthy and natural, it's good that she learns that.
Even if she seems fine I would still apologize to her. You didn't know she was home and it wasn't something you had intended her to hear. Maybe make a plan for if she gets dropped off like this again what she should do to notify you. I would emphasize it is not her fault because she is a child. I don't understand why the ex didn't tell you what was going on when he left her there.
I would apologize and tell her that if she needs to talk about it you are there for her. Nothing more. So you can follow her lead according her needs. I think that no one needs to hear parents having sex to understand it's healthy,... That's not ok for kids to ear their parents have sex. You didn't made it on purpose so be gentle on yourself. I would say something like "I'm sorry you get to ear your stepdad and I have sex. We didn't know you were home. Do you want to talk about it?" If she say yes, go on and answer her questions. If she says NO, tell her that you are there for her if she wants to talk anytime.
Aww dont worry about it too much. She is a teen and im sure kids at her school are much worse and probably don't stay with the same partner. She isnt oblivious, she knows you are having sex with your husband. It is the noises and image of what is causing those noises. At least she isnt walking in on it. Even though you arent with her bio dad that you being with your husband is normal for her. Don't over think it. We are human