Well, obviously they aren’t waiting for ring before they put out. And since they are pregnant, no worries on knocking them up. Plenty of horny guys would totally hit that, at least once.
(We see you're part of the broader cultural "let's-continue-to-impersonalize-women-as-sex-'things'-club"...May God convert your warped worldviews)
An STD when pregnant wouldn’t be a good idea and they would be taking that risk.