I think the reason it's hard to get a straight answer is that there's no broad consensus on what it means to have a waifu, metaphysically speaking. Some people believe their waifu is a literally real person who exists in an alternate 2D universe; others hold to the theory that ALL human love is ultimately for an idealized fiction, that this is no less the case for regular 3D relationships, and that the waifu crowd is just more honest about it; still others hold to the "common sense" view that we're just sad, lonely folks doing the best we can to eke out companionship in our own media-saturated imaginations.
"Waifu" doesn't necessarily correlate to "wife", either. Some waifus are wives, but others are simply friends, or sisters, or even daughters. And yes, some are worshiped as goddesses. And the concept isn't limited to one gender; there are a handful of folks on /mai/ with "husbandos".
The question of sex and virginity (on both her part and ours) is both huge and hugely controversial. Suffice it to say that a lot of us have strong opinions about whether or not it's ethical to fap to one's waifu, whether or not fapping to a fictional person counts as having sex with them, whether or not fapping to other characters constitutes cheating, etc. And Haruhi help you if you bring up any sketchy things like NTR or guro.
Long story short, it would be easy to just flippantly define waifu as "imaginary girlfriend" and dismiss the whole concept as ridiculous and sad (and many, MANY people do, obviously). But it turns out to be a surprisingly complex topic that isn't really susceptible to a brief, definitive explanation. The best I can recommend is to hang out around /mai/ for a while and get a feel for it yourself.