Leave them to it.

This is all getting a bit too much for you really. They're clearly into some kinky stuff and you've already knocked Belle up, you've not got any money involved in the situation and it seems now is an easy time to cut yourself off. The situation seems only likely to get messier, you don't want Belle to start making more demands from you. Maybe you'll fuck her again, maybe she'll even want another kid one day, but for now you don't think you should escalate anything. You've done your job, by getting her pregnant, even celebrated it. Let's not take any more chances.
With that you subtly creep out. Of course you've still left your wallet in there so once you're outside and in your car you send Belle a message asking her to bring you your wallet round when she can. You should be able to last a day or two without it you imagine.
She quickly replies;

"Sure, I'll bring it to you tomorrow, my stud ;) xxx". You hadn't really thought about that, but maybe you'll be able to get another go at her sooner than you expected.