To not want this sleepover?

[ChemistryBoggle]

Ds 16 yrs wants his gf to sleep over. Previously it was an easy no as she was 15 and I could use the excuse of not encouraging under age sex.
They are now both 16 and itnhas come up again.
I am uncomfortable with it. When insaid this to ds he said my discomfort should not impinge on his freedoms/rights/enjoyment.
Should add, ds also shares a room with other ds. 14.
I can't articulate why it makes sense uncomfortable in a way he understands.
He raises the point that if either other dc wanted a sleepover then i would say yes no hesitation, which is true, but they're 1) younger (but still teens) 2) not in a relationship 3)the same sex.
On point 3) ds points out that he is bi, so would I say no to another male sleeping over. No, probably not, but again, if in a relationship then yes?
I don't know. Aibu?
Please help me make sense of the reluctance!

On the basis that he shares say no unless his brother is away. If he had his own room I would have said yabu. I let dd from 16

Btw don't let the asd be an excuse for his behaviour - one of my DD's is autistic and she certainly didn't get away with talking to me like that, also I have medical power of attorney from the court of public protection because when she's in poor mental health, a few times a year, she cannot make good decisions

@Ellen888

OP, We still haven't heard what the girl's parents think about this idea and what his father's input is, could you clarify?

Apologies all, I was at work today. To answer the above, 8 have no idea what her parents think. I have met her dad once or twice in passing, and cannot recall meeting her mother except at open evening briefly said hi.. I certainly couldn't pick them out of a line up - totally face blind! We've never discussed their relationship for sure. I did mention earlier him staying there has never come up. The answer was always going to be no, as I said previously, it was more a case of how to say it. Re DH, they have a fractious relationship. Ds certainly doesn't tell him things like he tells me, they argue a lot. This thread has been super helpful, so thank you to all who have posted.