Nick really wants people to believe that he is some Chad, where he can just steal men's wives whenever he wants. He managed to get a mistress from a polycule, and now he thinks he is some alpha male slayer. This isn't even the first time in the past few months Nick has made similar statements about other guys besides Aaron, that he could steal their wives or girlfriends if he wanted.
This just feels like some weird delayed juvenile lashing out, where he always wished he was the cool football player in highschool or college and could get whatever girl he wanted, he could steal women from other guys, and he would stuff nerds into lockers. Aaron in this case is his projection of the 'nerds' from his youth. Nick didn't even peak in college or highschool, he just wishes he did. It would honestly be sad if Nick wasn't so evil.
Also, I am sure talking about stealing other people's girlfriends and wives (if he wanted to) really does boost Kayla's self esteem. Really shows how "strong" their marriage is now.