It's such a huge market, my sister used to work at a publishing house and did marketing for the romance novels and the numbers they did was crazy compared to other genres.
Indeed. It's insane seeing just how much romance dominates ebook sales, especially under Amazon's Kindle Unlimited program. From a business perspective it doesn't make any sense to write anything else.
Oh. That's actually qutite interesting. Perhaps try to focus on sci-fi romance and stuff and then move on to "pure" sci-fi/fantasy from there? People enjoy new ventures by authors they like, in my experience.
If it was any other genre I'd agree, but romance readers are
very picky. I've done it before, but the amount of readers I lost shifting from New Adult/Contemporary to romance with science-fiction elements was staggering. Evergreen niches like billionaires, bad boys and bdsm will always have a largest audience in Romance.
There's also the sad fact that sci-fi and fantasy authors don't make that much on Kindle (unless you have a publisher behind you, or you happen to be JK Rowling

)
Every time I open up my fantasy project in Scrivener, I think...do I write something I love, or stick with a genre that's making me millions every year?
Can't you mix erotica with fantastic / future settings and try to build a reader base that allows you to diversify your writing to get the opportunity to do a bit more in the field you love?
I already do

. Alien man-hunks, vampires and shifters were quite big in 2014-2016 and I rode that kink to the bank. It was a nice break from the usual shit and I had a lot of fun pulling ideas from Buffy, Star Wars and Firefly.
Shame that niche has dried up.
Is it so good that I don't need porn anymore?
Don't be silly. Everyone needs porn.