There’s a whole lot of difference between:
Dd11 “I think I’m gay”
Dd16 “omg your a fing queer”
DP “nothing wrong with being a lesbian- your mum had a girlfriend at 11”
In which case I think you just need to own it. Would it have been better for DP to say “I had a boyfriend at 11” yes but that might not be true. If Dd11 feels able to confide in you and her stepdad (is he her stepdad) about her sexuality then your doing this parenting thing ok. Dd 16 needs a chat on respect.
However if it was more like:
Dd11: Lucy at school saw me kissing Janie and told everyone I’m so embarrassed
DD16 “omg your a fing queer”
DP: that’s nothing, your mum was seen doing xxxx with xxxxx at 11
Then that’s really worrying. Maybe trying to make Dd11 feel better but more him having a laugh at both or your expense and would worry me about the blurring of sexual boundaries in your house. However if DP saw it was wrong and it was a one off and was truly trying to make DD11 feel better and apologied to all of you and you had a big family chat about boundaries and respect maybe forgivable
If it was anything like:
Dd11“xxx touched me in a way that made me feel bad”
DD16 “omg your a fing queer”
DP “it’s normal your mum was abused by xxxx when she was 11 and I still wanted her so that’s ok”
Then you have a big big issue. And I’m so sorry.