Several things
There is no superstition about kids dying because of their dfs lack of a BJ but there is a general superstition about not tempting fate by making jokes about your kids dying.
So that would make me think twice about my dp as a father in a wtf way.
I would also think twice about ever giving him another BJ. I love giving but I hate being asked in a sleazy way and that is just beyond sleazy.
Thirdly the whole ha ha I was only joking can't you take a joke after saying something out of order is a bit bullying and boundary pushing.
If he didn't know that would make her uncomfortable then he is an insensitive prick if he did know then he's a bully.
It is a bit coercive even if he doesn't realise it. Seems like his thinking is I really want a BJ how can I communicate this with maximum impact rather than I want a nice time with my partner what can I do to get her in the mood.
And finally this bothered her enough for her to tell you. And telling someone your dp has done an embarrassingly dickish thing is quite a big step in itself so she is really bothered.
And it has really bothered you OP.
So....I think he is an arse and I'd not be waiting till she brings it up again as if you don't she might be too embarrasssed. Next time you speak I'd be saying you know that thing your dp said, it really got me worrying because it isn't right. Are you ok? How do you feel about it now? And has does he often say or do things that make you uncomfortable or is this a total one off? Because he never seemed the type to me but I know the public face doesn't always match the private personna.
Just so she knows you took it seriously and if there us an issue you will believe what she says and not the public impression he gives off.