I always wondered why he just didn't talk to her about contraception. Would of saved my man alot of time if he just had a brain and said if we do this no children. Easy.
Ok, time to turn off my coomer brain now
My take is that, rather than him not telling her, it's more like something (left out) for the sake of plot.
As we know, in 3 years she's been in the modern era/world, she had multiple occasions to learn about them: she watched porn, she had the power of google (or whatever the chinese search engine is), she went out with ping ping (i think this is the other girl's name...maybe?), she went shopping or groceries shopping (i think you can find condoms in a konbini like market or simply in a pharmacy).....
It could be something that was left to the readers immagination, as it not just about them: we also don't directly know how she does for sanitary pads, cramp meds, period meds....heck, we don't know about how she deals with her period.
That's why I think it's one of the details left out just for the sake of plot or not dealing with too many small things in the story