I think the thing with Janice was all political. Richie I believe used to date her in high school (so maybe she was hot then?) so he might have had an excuse to get back with her (although there's no indication if they were seriously dating or if it was just a fling), but even that was so he could get closer to Tony and demand his old stuff back since he was out of prison. Of course it's implied that Richie is bisexual or at least had some homosexual encounters while in prison (when he gets out and they give him a party at the Bing, he has the lights off while the stripper gives him a blowjob and when asked if he likes it in the dark says "Yeah, something like that", and takes major offense at the notion his son might be gay.)

Ralphie was almost certainly just doing it to get into Tony's good graces since he was still in the doghouse from beating Tracee to death and being a general scumbag. Also given the shit he was into maybe Janice was one of the only people who would do it to him.

I think Bobby might have had genuine affection (wasn't his deceased wife somewhat plump too? I can't remember) but again the relationship was more for power and to get an "in" with Tony than because Janice was some great catch. The king's ugly sister is still the king's sister, so you put up with the fact she's ugly because it gives you a political edge.