I also tried to follow the kids’ lead. I’d ask if the boys wanted to invite friends/GF to a meal with us when we visited. Kids could pick who got to come. I was courteous and curious and tried not to pry too much. I was lucky and the boys DID want us to meet GFs. We took everyone to somewhere fancy and ordered tons of food.
Visits to us? I asked how many rooms to make up. We did let them stay together. No GF invited on vacations, not yet. We are getting close to that, though, with a more serious relationship for one kid. GFs came to grandparent homes when a holiday was celebrated there. No sleeping together at grandparents house!
Most of the sex talk was in HS. I may have reminded them once or twice in college to be careful and use birth control and also to be careful about having sex when drinking.
We met parents at graduation. It made sense because of the occasion and because the relationship had lasted so long. We follow each other on FB.
I try to be welcoming and friendly and find out about the GFs. If GF was present at a gift giving holiday, I’d get her something small so she would be included. Generally, as others have said, I treat each GF as if she might be “the one” but also don’t get too attached.
(My own MIL was not good with boundaries.)