There are a lot of things that are forbidden fruit to best bros (or even close bros), the list of these verboten acts constitute the bro code. While the written Bro Code is a contrivance of How I Met Your Mother's Barney Stinson (played awesomely by Neil Patrick Harris), unwritten (and largely unsaid) rules exist between real life bros.
Almost all of these unwritten rules involve the acquisition or retention of tail. There are several understandings between gentlemen that all but make moot the phrase "all is fair in love and war." Chief among them is the idea that nuclear family is expressly off-limits for carnalized exploration. While there is some wiggle room involving sisters (if actual love and affection are present), but mom's are a strict no-fly zone (there are extenuating circumstances in which stepmoms are in play). That being said, please check out the below video (if you're in the office, earphone it, if you're in front of kids, earmuff them, if you have a pet project, earmark it):
As always, JT and Andy Samberg are outstanding (as are Patricia Clarkson and Susan Sarandon, nice pick-up getting those two) but the lesson (like d*ck-in-a-box) is that these two characters are really bad decision-makers. While mutually being a best bro's stepdad ("It would be my honor to be your stepfather") seems like a good idea, it will only lead to resentment (seriously, does any guy like the idea of someone shtupping their mom, even if they are "cool and thoughtful"?).
In summary, dudes do (and don't do) a lot of weird things out of a sense of propriety and brotherliness. Make fun of it if you will, but it's really the only thing that keeps us from flinging feces and pounding our chests when we imagine that our territory is being crept-upon.