Here's the thing - it's very unlikely that for a normal average couple, a little saliva will harm their chances at all. I mean, since the dawn of time saliva's been in use pretty frequently, LOL.

But if a couple is already having trouble conceiving we'd want to remove any possible issues that might be lowering their odds. This doesn't translate to then that everyone needs to avoid saliva, just that it has been shown to kill sperm so anyone who's needing to improve their chances will want to skip it (and anyone swaying blue!)