So you will have mostly a group of 20 year olds, who are willing to answer a lot of questions. It doesn't matter how big the pot is, thats not a random sample of society.
Is that all? That doesn't make it "incredibly" biased.
I mean, looking at your example of "black women being the least desirable on OkCupid", it's easy enough to check if it applies only to 20 year olds who are willing to answer lots of questions or if that correlation actually exists outside of that group[1].
Your way, of simply dismissing it instead of regarding it as a data point that can be further investigated is the opposite of science.
[1] OkCupid certainly had the data, and on one of the blogs I recall seeing their distribution of ages, and it was not dominated by the 20-30 age group, even if they were a majority. It would have been easy for OkCupid to redo that desirability analysis for particular age groups.
The classic illustration is the Literary Digest poll of the 1936 Presidential Election. They got over two million respondents and confidently predicted a heavy Roosevelt loss. George Gallup got a comparatively tiny but well sampled set of respondents and correctly estimated the Roosevelt victory. The magazine's problem was partly sampling bias (their readers were richer than the average American, amongst other things) and partly non-response bias (they got a return rate of ~25%, which is pretty good, but probably disproportionately from the most politically engaged of their readership, who hated Roosevelt).
I don't know how bad the sample is here for extrapolating to 'all those seeking dates', but the size is a distraction - the sampling is all.
(This isn't necessarily true if you're making individual profiles, searching for rare phenomena, or something else which is more in the 'big data' than 'social statistics' space, and which genuinely needs that much training data. It's also not true if you're studying the site members themselves, for whom the site members are a perfect (non-) sample. The latter is true for the in-house data science team themselves, but we shouldn't just assume their experience will extend to the rest of the world).
I gotta ask, in case what way is the sample of a few millions of people who want to date 'incredibly' biased?
Slightly biased I might buy, but how on earth can you make the leap that people looking for dates via a matchmaking site is not at least slightly representable of people looking for dates?