This is exactly what my sister went through with a guy that said he wanted to marry her "eventually". She started to find on-line dating sites and even a newpaper clipping of a dating club and other similar info.

He ended up cheating on her and there was proof which I won't go into.

I agree with most of the above. He's either cheating already or will. You deserve better! I hope that whatever you do, it will work out for you. But if you're asking...I'd say "run" as another above has said.