When I get feelings like that I just remember all the good times and good conversations I have had with my girl and it makes me forget other attractive women I see. Sure I could probably leave my girlfriend and go try to have sex with someone who might look better but how likely is it that they are a perfect fit for me and have all the same ideas about rasing kids and have the same goals as me. And in the end everyone is all wrinkled and old so why not get someone who will sit on the porch and talk with you for hours.