"If they can't interbreed with humanity, how to these different species met up and have children in the modern world? Is there a cryptid Cupid.com?"
Yay, cryptid breeding! We're going to restrict ourselves to a very anthrocentric approach, for the sake of answering the question as posed: we're only looking at cryptids that can, for one reason or another, live in human settlements. The ones who can "pass," in other words. They're both the ones who are most likely to have issues with humans seeming attractive, and the ones who will have the most "I need to know your species before we can hook up" problems.
Some of these cryptids get around the "accidentally dating humans" issue by not being mammals. For the most part, humans smell "wrong" to them, and are hence not attractive. You do get occasional perverts who like mammal boys or insect girls instead of sticking with good, honest reptile people like their siblings, but for the most part, people are attracted to things they have half a chance of being biologically compatible with. (Science supports me on this.)
A few species of cryptid are cross-fertile with humans. A very few species, and most of them will still choose not to crossbreed, because there's a very good chance their offspring will be infertile, which doesn't help keep the family line going. Lilu, which includes both incubi and succubi, are cross-fertile, and their offspring have a fifty percent chance of being infertile. Fertile crossbreeds will usually have children that are indistinguishable from the species of their mate. So if Elsie, who is half-succubus, were to visit a sperm bank and get some human sperm, she would have a baby who was effectively a very sexy human, rather than a succubus crossbreed. Jinks and leprechauns are similar, and many family links have a little jink or leprechaun blood.
Tanuki are the only known species of therianthrope to be cross-fertile outside of other therianthropes, and they manage this partially by having lots and lots of tanuki babies. As with certain types of frog (no, seriously, science again), they have evolved extremely dominant DNA. If Ryan were to mate with Istas, you wouldn't get half-waheela babies, you'd get unusually colored tanuki. Even when there were more of them, they would often seek out-species mates to get hybrid vigor back into the community. Sorry about what that does for your family line, dearest.
There are dating services for some species of cryptid, ranging from the very communal "let me introduce you, he's a nice boy" social dances of the bogeymen to the more formal courting rituals of the gorgons. There are very few true solitaries, and almost everyone knows where everyone else is. And yes, some of this happens on the internet, although it has to be very carefully masked and monitored; the Covenant can log on, too.