As someone who experiences both (...and even limerence unfortunately): It's different for everyone. It's tough for me as an autistic person to sort it out. How are you supposed to describe feelings with words when the difference between them is minor, nuanced, and still somehow huge?

Personally sexual attraction is pretty straightforward. Someone's physically attractive. Might not immediately be WOW I WANNA HAVE SEX WITH YOU but combine a physical presence and, uh, yeah. Sometimes thirsty brain's gonna thirst.

The trouble with romantic attraction is if I'm romantically attracted to someone, sexual attraction kinda just happens and I wonder if it was already there or not. First time I realized I was bi, it was after years of a crush on a close friend I didn't even realize was a crush. It was recognizing the sexual attraction that made me realize what it was. What was it while I was filtering that out? I idealized them, I wanted to be more like them and pretty much saw them as a better person than me, being around them and helping them out was emotionally fulfilling. ...Then it was still that but also I got the butterflies in my stomach and oh, shit, they're super attractive actually. Oh, shit, existential crisis bus ride.

Sexual attraction, I want to "sleep" with them. Romantic attraction, no seriously I want to sleep with them regardless of euphemism.