Sounds like strange priorities to me. You mentioned your partner (not sure now, what sort of partner is that) only as a background and don't seem to think about her feeling, etc. Or did I get the wrong impression? You don't cheat on someone you trully love. Why are you still with her?!
Untrue. Love does not divide, it multiplies. Trust me, I don't love one son less because I fathered another one. I love them both very, very much. You CAN love more than one person at a time (even romantic love) but it's LIKELY that you're right in that it's lust or infatuation with one of them... but it could be true. Lets not be judgmental about something we can't know for certain.