Emma is a good period romance.
Dear Mine is extremely shoujo, but one of my favorite romance manga.
Mars is very heavy but undeniably good.
Kanata Kara is your classic "adventure with romance" story and is quite excellent. Shoujo of this type usually just plain suck (Fushigi Yuugi anyone?), but this is the exception.
Three of the four of those are not of the demographic you requested, but well, shonen romances are almost always awful and there are so few decent seinen ones...