Elin is unavoidable if you have sex with her, but you can still refuse her right before crossing that line while entirely romancing her otherwise.
I just checked - and it's hard to say if the romance with Elin continues after refusing to have sex with her in that moment. The dialogue in 1.1 is very... neutral.
The conversation between them immediately after refusing suggests that they both agree to stop their relationship for good.
All I know is, refusing to kiss Seraphina (on the Love path) ends the relationship then & there, which is why I think the same applies to Elin here - you can't continue her romance if you don't have sex with her, and having sex means she gets pregnant.
So no, romancing Elin means she always ends up pregnant. The same goes for Seraphina, and these are the only 2 women you can't avoid knocking up on their romance paths. So far, Cass and Lia are the only major LIs that you can avoid impregnating while continuing the romantic/sexual relationship normally otherwise.