Yosuga no Sora, e.f., and valvrave, Also aku no hana (I think, never saw it myself)
I agree the way anime represents romance is generally juvenile by typical standards. That said I personally like the innocent and pure nature of it. Romance in western live action media sickens me the way people use and abuse each other, play mind games, and treat each other like disposable sex toys. It does bother me how relationships often don't go anywhere in anime, often ending on ambiguous notes. That and how people tend to beat around the bush endlessly, but it's also cute and kind of nice.
"Mature" adult romance means throwing yourself at countless random people till one agrees to let you buy them food, watch a shitty movie together, then stick your dick in them after hanging out for a day or two. All while lying your ass off to them to make yourself seem more interesting than you really are, just so you got someone(anyone) to stick your dick in for another night. Then you keep doing that until you find a more appealing sex toy willing to let you fuck them too, at which point you may or may not tell your current fuck toy you're not gonna fuck it anymore, and upgrade to the slightly better fuck toy.
Anime romance is good honest strong slow building real love. It involves people who like and care for each other being close, enjoying their time with eacother in an innocent platonic way, and not rushing anything. Sex in relationships like that has actual meaning, it's making love with someone you love, not ramming your junk in a random person. That's something I find far more mature than mindlessly fucking like animals.