Your analogy simplifies things too much. A co-founder leaving isn't at all like breaking up with a girlfriend. Many other people are affected in the situation.
I think marriage is a more appropriate comparison. Divorce affects your immediate (investors, employees) and extended family (users, partners, other stakeholders). It's a decision I know I'd think about a great deal more than a break up with a girlfriend.
Right, which is why I talked about marriage in my analogy. A deeper commitment means that you ought to give it some time to see if you can shake the loss of passion.
In the case of a 3-5 month old startup, though, with no employees (dunno about investors), I imagine it's a bit closer to a girlfriend than a marriage.
I think marriage is a more appropriate comparison. Divorce affects your immediate (investors, employees) and extended family (users, partners, other stakeholders). It's a decision I know I'd think about a great deal more than a break up with a girlfriend.