What's the male equivalent of 小三、二奶, etc.?
The closest thing I can think of is 小白脸 but it seems to be missing that certain something...
A proper term would be 情夫:
小三 can also be applied to a male as it means the third party in an affair or 第三者.
二公 and 二爺 are informal terms opposite to 二奶.
小白脸 does not necessarily refers to a man who has committed adultery with a married woman. It can also mean the the guy is living off the woman or that the guy has a fair complexion.
Or be more formal, "情夫".
PS: "小三" can also be used for male in theory.