It's gotten more complicated now that the concept of romance has been expanded to include multi volume arcs where the couple doesn't have their seminal HEA until the end of the last book.
Oh, I think a series is fine, as long as there is a HEA at the end of the series. I'm thinking of things like 50 Shades of Gray, and maybe Twilight.
I noticed that some that are "hybrid" genres, like SFF Romances, can sometimes have series that might start off with HFN endings at first and then finish with HEA at the very end of the series.
SFF stuff tends to have other plots going on along with some, or a lot of, worldbuilding, so that could be why some (or many?) of them expand into series at times.
(This is different than SFF and Mysteries that just happen to have some romantic subplots, some of which don't end well or never go anywhere, etc.)