I'm curious if anyone else has had this experience. I say this because in general it seems easier for most to figure out ones own sexuality first, than their romantic orientation. I can't recall reading anyone having the reverse happen to them.
I'll share my own experience with this:
I thought I was on the ace-spectrum until fairly recently, since I wasn't really that interested in having sex. But I realised this lack of interest was not caused by lack of sexual attraction, instead it was mainly because of being both touch-averse and socially anxious. I can desire sex - it's just hard to meet the right conditions to the point I would be comfortable enough to act on those feelings. It was easier to believe I was ace so that I didn't have to confront it.
It also took me a long time to reach this conclusion partly due to my confidence in being aro, it was easier to assume my romantic/sexual orientations were aligned. And because I had no interest going into romantic relationships, I didn't really have an accessible way to explore my sexuality in a hands-on approach (I'm way to nervous to try anything with someone I don't know).
Oh, and one more contributing factor is that I discovered aromanticism through asexuality - I had connected with the idea of being ace first, thus I did not think of going back to re-evaluate until much later.