Boyfriend who hangs with pedophiles..

We are 18 with a baby on the way. Been together on and off 4 years. He has a few friends but two in particular I don't like. They are about 19 and sleep with underage girls. I recently found out they had a 3 sum with a 13 year old. Which my boyfriend told me himself. One of them also asked my sister who is also 13 (which he knows) to give him oral sex. He says he only sees them occasionly when they are with mutually friends and doesn't exactly "hang" with them. He says he can't help seeing them as they all have mutual friends which I know. but I really don't like that he has anything todo with them at all. I know I can't tell him who he can and can't be with. I haven't said anything to him about it.. do you think I should? It just makes me really uncomfortable