She did date other guys
including Rusty, but they've always let her down. Who's to say that, after one such disastrous date,
maybe the Rusty one, Jill came crying to Bella who comforted her and, at the time, Bella was also dealing with her own feelings about the break-up of her marriage. Then, after a lot of consoling and wine drinking they went to bed, gave each other a goodnight kiss, and it all just went off like a powder keg of sexual frustration, desire, and loneliness.
Kenderlovag 09 also had an interesting theory that, if Bella had feelings for Jill's sister that were either unrequited or requited but didn't work out, then maybe, in that moment, Jill reminded her of the sister she once had feelings for and that made it easier for Bella to give in to that moment.
Also, I don't see why DPC couldn't do this because, for one, the M&J and B&J situations are entirely different because one deals with two people who were in a committed relationship and the other deals with two friends who had, at least, a one-nighter and have done their best to move past it. They deal with different issues and different emotions, both of which are interesting to explore. Also, after what happened in AL, maybe DPC is just having a bit of fun here by giving players 2 options for a threesome relationship, and instead of killing it off, he's going to make us choose between them.