Before I talk about myself, I just wanted to point out that the idea of splitting up attractions is something we're used to the the ace and aro communities, but I feel we often forget that it may not apply well to others. Not everyone's attractions divide up nicely, and they may not divide up at all and be very interconnected with each other, hence it seeming like many people who experience romantic and sexual attraction can't seperate out the romantic 'noise'.
On to talking about myself, I don't think I spontaneously form sexual attraction towards people, but it's a little difficult to know. This is because I usually don't realise that I'm attracted to someone until they appear in my imaginings, either in a sexual way or a particular kind of intimate-friendship way (since I like the idea of having sex with people I know well). So I don't usually know when the attraction started. Another thing is, I also somethings 'borrow' other people's attractions so sometimes it can take me a while to figure out if I'm actually attracted to them, or if I've been in an environment surrounded by other people expressing that attraction and picked it up from there.
I know there is a difference for me between being attracted to a person and actually wanting to engage in sexual activities with them. The only one I know for sure, though, is the intimacy one. I can experience sexual attraction to someone I don't know and can happily imagine having sex with them, but I wouldn't want to actually have sex wth them unless I was close with them. Unfortunately, if I get to know someone my brain has an unhelpful habit of sort of 'friendzoning' them, so the attraction rarely survives the process.
I definitely had some other stuff to add, but I completely forgotten I'll add them later if I remember.