I Said Yes, But Now I Am Nervous.

I'm a single mother with a 19y.o daughter. She's a lovely young woman who is very sheltered because she is borderline intellectually handicapped. She's only had one boyfriend, but they've been together for almost two years, and they're totally in love.
I'm positive they both virgins as he is intellectually handicapped, and she tells me everything. He's almost 21, and so far all they've done apart from kissing and cuddling, is masturbate each other to orgasm. He is planning on moving in to her room in my house (with my permission), days before his 21st. This is just a couple of weeks away. They're fine, they both have jobs and they do very well despite not being as intelligent as their peers. They don't need care, and can manage their lives just fine.
Recently, my daughter told me that she wants to have sex with him on his 21st birthday. I've had her on birth control since she was 16 as a precaution, because I seriously thought that she would soon loose her v card to some random kid at school. She didn't. I'm quite the proud mother, she's done much better in life than I had at her age (I had her when I was 18). Now here's the part that has me nervous (and I already agreed to it). She very frightened and nervous about actually having sex, and I have spent hours trying to explain things to her so that she's aware of what to expect. She's decided that she is going to do it, but has begged me to be in the room, in case she needs me. WTF? I have tried everything, telling her I would be almost there in my own room (we share a wall), and she can call me if she needs to. I have suggested that she calls me on her phone if she get worried about anything. It doesn't matter what I suggest, she's convinced she needs me there. I have told her it's not normal for parents to be present when their children loose their virginity. Even that her boyfriend won't be comfortable with me there, but she's assured me that even he would prefer me there in case he does something wrong. No amount of explaining how it's natural and instincts will take over, or that she really doesn't want to remember her first time with her mother in the room. Nothing I have said has persuaded her to leave me out of it. She's convinced she needs me there. Although I find it super creepy and totally uncomfortable, it's not enough to get out of this. I agreed while hoping that I could convince her that she doesn't need me there, or maybe she'd change her mind, but it doesn't look like that's going to happen. I'm kinda stuck. The lights will be on.
Any advice appreciated. This is getting serious and I don't have much time, or a reasonable excuse to not be in her room.
Thanks for reading, I hope someone sensible can message me soon.