Auron,

It sounds like this guy is experiencing typical "married" relationship issues, and you are setting yourself up to be the "other man" . . . I think you should really ask yourself if you are making yourself available to him so that you can be his new boyfriend. If that is the case then it is not a good habit to get into. Many people refer to this behavior as "stealing" or being a "home wrecker".

BTW, most open relationships assume that sex outside the relationship is just about getting off. If you have a significant relationship with this man that you culminate in meaningful sex then his boyfriend will probably consider it cheating.

My advice is to let the relationship end before you start fucking, which I think your instincts are telling you is right thing to do.