Potatoes that is not a view I subscribe to. The op KNEW he was in a committed supposedly monogamous relationship with someone else AND there's a child there too. She is no less responsible for this mess than he is.
It's not hard NOT to shag someone who's committed to someone else. That goes for men and women.
I'm 46 and have had more than my fair share of sexual relationships from fwb to fully committed and I've managed to avoid doing this, as most people do!
In the majority of cases it's bloody obvious if someone is already in a relationship with someone else. We see threads on almost every day certainly several times a week where it's obvious to those of us reading (and I suspect deep down the op's) that the odd behaviour of men who:
Don't introduce op to friends/family
Don't have op coming to their home
Only see them on certain days/times
Don't encourage indirect communication at any time
Are reluctant to spend full nights at op's home for no obvious good reason
Avoid conversations about future progression of the relationship
Usually are quite careful even paranoid about contraception
Secretive in any number of other ways...
There's even numerous threads about OLD where clues to a subscriber possibly already being in a relationship are discussed.
As for the sti aspect there are very real risks to the pregnancy, the mother and the baby if this aspect isn't considered, I've seen women devastated by problematic pregnancy & birth or even losing the baby as a result of undx sti's.
Personally I think all pregnant women should be very strongly encouraged to have full screening as soon as pregnancy is confirmed.
Some can lay dormant for several years and to be quite honest many women aren't in tune enough with their bodies or aware of the possible symptoms to recognise they may have an infection which can pose a risk to their baby.