Is it just me who's tempted to suggest the exact opposite advice? LW you don't need to cut off their son, but you do need to cut off the parents, and you don't need to be coy about why. It's not that unusual as a kid to have friends whose houses you never visit but who can come to yours, and if Thomas won't allow that, well, that one's not on you.

Meanwhile, Thomas doesn't seem to be treating his past as a horrible unspeakable secret, so I vote you just carry on and pretend it's the interesting anecdote about a shared past that your husband thought it was going to be when he brought it up. You turn down an invite, someone asks why, you say, "Funny story, actually, my husband went to the same college as Thomas, and I guess he knows too many details about Thomas's time there to just brush it off. What details? You know, the big rape case he was involved in, he wasn't convicted but it was pretty obvious if you were following the story, and there's all those later allegations--" or "With the work I do, it just doesn't seem right to be seen around an accused multiple rapist - oh you didn't know? Yeah it was all over the media when it happened, I don't think it was ever a secret--"

If he does try to make drama about it, you go, oh, ooopsie, I didn't realize it was some big secret, after all you were happy to chat about it with my husband! And in the press!