Wow, I'd be seriously surprised if that was really what the paintings actually meant. Considering that cave paintings were from a time long before any domestication had occured, you can imagine how fkn dangerous it would have been to have even attempted such a thing. Having sex with a domesticated goat is one thing, but you've gotta give props (and I'm not into the bestiality thing) to anyone that would try that with a wild animal from ~64k years ago.
TBH, a modern human having sex with one of those cavepeople would in my mind be little different to bestiality.
hrimthyrs' point notwithstanding, if you are using a time machine, it wouldn't be bestiality because they're still human, both Cro Magnon (almost indistinguishable from modern humans) and Neanderthal (many of us have some Neanderthal DNA--I think I have 2.2% if memory serves).
If anything, wouldn't these be the ultimate in incest? Impregnating someone who is potentially your great, great, great,...great, great, great grandmother (somewhere between 400-1600 greats)?
We had the movie 65 which puts Adam Driver's character 65 million years in the past. How about something around 10,000 years or so, when Europe had both Neanderthal and anatomically modern humans? How a single modern MC could mess with genetic history...