I need a few Christian pastors with a lot of experience to answer this for me please.?

This person I was dating a while ago before I was saved got back with her ex-boyfriend, which whom she has a child with. She tells me she lives with him but does not do anything sexual with him and has no relationship whatsoever. I think she is lying to me to get into my pants, I'm not bragging or anything but I'm good in bed and well endowed. My question is, if I get back with her am I sinning, I have the thought in my mind that they might still be having sexual relations and I'm just there for entertainment. I have stopped doing that with her for a long time now and I don't want to go back just to find out she's just using me for my body. Honestly, I'm lost... I've prayed about it so much and can't seem to find the answer, please help me.