I read a lot of fanfiction, but have tried romance too, and yeah, a lot of it is just bad. Personally there are words that a lot of writers seem to use (or over use) that just makes it unsexy for me, or they read like stereo equipment manuals in terms of how detailed they discuss how and where things are going (I know how tab A fits into slot B thank you), or they fail basic biology and state impossible/dangerous physical actions. Or even worse, they describe what would be considered SA as some sort of 'romantic' encounter with zero realizations that it's just SA, and it's not even kink related. I've read kinky stuff that wasn't my jam but still was sexy as hell, and basic vanilla stuff that made me so uncomfortable that I stopped reading. And even all that isn't discussing sex scenes in regards to plot, and how way too many writer's seem to throw one in like you'd add a dash of pepper while cooking just to 'spice things up'.

I'd say read reviews for sure, if it's fanfiction make sure you check tags, and if you find an author who writes a scene you like, try to find other authors with similar styles.