This is a weird contest to partake in. That they have invited girlfriends to watch before, elevating the contest to the levels of spectator sport makes it even weirder. 

What's next? Brazzers sponsorship and a live telecast for TV audiences?

You are not being sensitive or overreacting by not approving. This behaviour is not normal seen at all whether they are 19 year olds or not.

If you can't approve of it, you are better off going separate ways. It's not that you are dating someone younger. It's that you both have different standards for normal. You define normal differently.