Having the ex living with you would definitely scare away potential future girlfriends, in fact living with any females could potentially scare off any future girlfriends - I think men are totally clueless when it comes to the complexity and underlying issues women have with each other, it's not just about a potential rival in an ex or worry about you getting back with her, but it's a territorial thing too.
I'd say we feminine folk are more territorial than men, the difference is that while men show their territoriality with pissing contests and in more up-front ways, we women do it more subtly and that subtlety can be hugely damaging and so something you really want to avoid, most women wouldn't come within ten foot of you with an ex living with you, I know I'd even think twice if you were living with any female.
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Bloodheart
Good-looking girls break hearts, and goodhearted girls mend them.