Presuming that all 16 year old's start having sex the moment they turn 16 (they don't), then this girl has been sexually active for around ONE year, whereas your Partner has SEVENTEEN years sexual experience (the entirety of her lifetime) under his belt. I find it highly unlikely, that she was the sexual predator in all of this. FGS, she might even still be at High School.
I have a 17 year old daughter, and all of her friends are aged 17/18. Only 3 of her circle of around 10 friends, has had a boyfriend. The other 7 have not yet had a boyfriend, and would not know how to be sexually aggressive if they had the handbook.
In any case, even if she HAD been the pursuer, he SHOULD HAVE DECLINED, because he has a Partner.
I find it worrying also that you have been together for 10 years and have 2 kids, and he is yet to marry you. Maybe an old fashioned view, but speaks volumes about his commitment to you.
What is also frustrating, is that if you do split, I am sure that you will be left with the lions share of caring for 2 special needs children, whilst he will be free to swagger his stuff with the "laydees". How fucking unfair! Make sure he pays Child Support and has your kids a few nights a week, so that you can get a break, and eventually start dating again.
Believe me, he will regret this one day.