Actually, I've been debating that little fact with a friend of mine. Every time they talk about Alice's bonding with the T-virus, it's on a cellular level; DNA wasn't contagious, last I heard. To us, it was "live first, love later" that kept those two apart, as well as Alice's fear of endangering the others. I mean, hell, the best line between the adorable couple in Apocalypse is "Don't worry, I'm not contagious."