If I missed this in one of the other responces, I apologize. But it would be worth repeating.
Ok, so she was going to break it off with other guy, because he sucked in bed. BUT, somehow, he convinced her, that it was due to him not being able to be "emotionally" in to it. That he really cared, blah, blah, blah, and that he could only "Show" her how much she meant to him, ( As in, only fuck her good if ) If he was the only man in her life.
Your girl is in the throes of NRE. ( New Relationship Energy ) She is feeling all giddy, and the rose colored glasses are on. She is looking past the "Bad sex" because she feels "Loved". Feeling new love, creates a chemical release in your body. It makes you want to pair bond. Its natural, and if you were experienced in Poly relationships, you would know what to look for. Eventually, It will wear off. She will, hopefully, gently, drift back to earth, and her head will be out of the clouds. DON"T try to pull her back down. You will get burned for it. Work on your relationship. Do some research on NRE, and show it to her. Talk to her about it. And then be the best man you can be. She will either cut off you, in which case she will more than likely regret it some day, or when her feet touch ground, she will realize she is still getting "Bad Sex" and cut him loose, or you will figure it out together, and she will balance both of your needs.
Either way. There isn't a thing you can do about what she decides, except be the best possible man you can be. Acknowledge your faults, and fix them. Meet her needs when she is with you. Be confident in yourself.
Thats about all I got.