A good question. I think it might actually be an ideological thing: we can overcome species differences to love/marry/breed. Would be in keeping with Roddenberry's humanist approach.
Just a guess..
"War is an ugly thing but not the ugliest of things; the decayed and degraded state of moral and patriotic feelings which thinks that nothing is worth war is much worse. A man who has nothing for which he is willing to fight, nothing which is more important than his own personal safety, is a miserable creature and has no chance of being free unless made and kept so by the exertions of better men than himself."
John Stuart Mill