My Gateway Romances were Deanna Raybourn's Silent in the Grave, Nalini Singh's Slave to Sensation and yes, as heretical as it is to say it among some romance purists, Fifty Shades of Grey by E.L. James.
Before I found romance I read YA, Fantasy and Urban Fantasy for the romantic arcs. It wasn't till I became a romance reader that I recognized the pattern.
As I was finishing up Grad School and had time to read again I read Twilight at a friend's urging and later read her copy of 50SoG because she had read it and wanted someone to talk to about it. There was certainly more sex on the page than I had ever previously encountered with the possible exception of the Jean Auel books I had to sneak read in middle school. I read a ton of erotica and books with black covers and inanimate objects on the covers immediately afterwards looking for something that would capture my attention in the same way. Most of it it didn't click with me, even though they were certainly hot but I kept trying looking for that X-factor that had caught my attention. In the midst of all them I did find a couple of Charlotte Stein books that told me I was on the right track.
At about the same time I became aware of Felicia Day's Vaginal Fantasy Book club's. I was drawn to the virtual bookclub because identified with Day and the rest of the VF crew as fellow geek girls, women who had read a ton of SF/Fantasy and comics like I did. For someone who never seriously considered reading romance, their enjoyment of genre romance novels was a powerful recommendation. I watched the first half-dozen episodes and started checking out the books. The first two I read on Day's recommendation were Silent in the Grave and Slave to Sensation. They were immediately accessible to me. I had been a Austen and cozy mystery fan, so mystery series with a strong romantic core was in someways very familiar and it started opening the genre to me. Slave to Sensation with its much more overt romantic arc was a bigger leap but Singh's fascinating world building and fast-moving and suspenseful action plot, eased me in.
I immediately read the rest of both those series and as I read them I realized the thing I was looking for was strong emotional conflicts. Thankfully I had access to the NYPL and my local libraries eBook collections and they both had a wide variety of romance eBooks available. They had everything from Harlequins, e-rom, PNR and a gazillion regency romances. When I ran out of Psy-Changelings to read about I ended up trying Stephanie Laurens's Cynster books. The bossy, over-protective heroes in those books had a lot in common with the Singh's changelings. They even worked as a pack, and they were essentially invulnerable. They made it possible for me to transition from binge-reading PNR, which I was still sort of classifying in my head as sexy SF to reading "real" traditional Historical Romances of which my library had hundreds.
I eventually admitted to myself that I that I was a romance reader, not just a Mystery, Fantasy and SF reader crossing over Once I did I started exploring the RITA award winners, and seeking out romance blogs for recommendations. It has been about 5 and half years and my reading preferences have evolved as I was introduced to new authors, tropes and trends but I will always have a fondness in my heart for the books that drew me into the romance genre.