Very sweet movie.
Except for the Alan Rickman / female office assistant bit :(.
A Very Long Engagement
Somewhere in Time
Much Ado About Nothing (Ken Branagh version)
Two movies with tuba-playing women! Make it a double feature! 
Amen!
And if you will permit a slight violation of the OP’s “hetero love stories that don’t involve infidelity or serious angst” there are some thrillers that cross the border into romance such as:
A Place in the Sun (1951)
Blade Runner (1982)
Body Heat (1981)
Elmer Gantry (1960)
and if you can find it
Gotham (TV 1988)/
Yeah, infidelity is really off the table. Sad or ambiguous endings as well. And the love story should be the primary plot, the reason for the existence of the movie. For example, Body Heat is a favorite movie for many reasons, but isn’t a candidate here. Same with Love, Actually. Etc. But, many good suggestions … and additional confirmations. 
More always welcome.
Let’s see.
Steamy? Check
Protags 25? Check
No infidelity? Check, though something is suspected.
Not a formula rom-com? Check.
Lighthearted? Check.
Zack and Miri make a porno.
No, really.
I believe you! And I sorta wanted to see it anyway …
:smack: I missed the “don’t” in the OP. Oh, well…
Yes! Another “please don’t let the marketing make you miss this great movie” movie!
sad This rule crosses off a beautiful, lovely, wonderful movie that opens in theaters tomorrow, called Beginners. It’s half about the love story of Oliver (Ewan McGregor) and Anna (Mélanie Laurent), and half about the love of Oliver and his father Hal (Christopher Plummer). And a bit more (there’s the love of Oliver for his mother, the love of Hal for his lover Andy* and the love of Oliver, Hal, Anna and for the dog Arthur (Cosmo) thrown in). The movie is so full of love and joy, with some sadness, but not overwhelmingly so. None of it is cloying or sappy in the least.
- there’s no infidelity. Hal has a good marriage, never cheats, and doesn’t come out of the closet until his wife has died.
“A Matter of Life and Death” – by Powell and Pressburger, with David Niven, Kim Hunter, Richard Attenborough, Marius Goring, Roger Livesey. I noticed there was a thread about this one a while ago, here, which is good. I watch this pretty often, and it’s the only movie I’ve actually cried at for a long, long time. Good, clean fun. It’s actually pretty steamy for the time – I think Kim Hunter is nice to look at, and Kathleen Byron is pretty stern here, as a bonus, which works for her, I think (and for me). I would call it a pretty serious, moderately sexy movie with a happy ending.
Pride & Prejudice (the 2005 version) - This movie is why I’m borderline-obsessed with Keira Knightley; she’s luminous and irresistible in it. Great cast, script, cinematography and soundtrack, too.
Heaven Can Wait - Funny and sad and wry and wonderful, all at once.
Groundhog Day - One of the best romantic comedies ever. Sweet and sarcastic in just the right measures. Best seen in February, obviously.
Love, Actually - Another great BritRomCom, and not just because, ahem, Keira Knightley is in it.
While You Were Sleeping - I haven’t seen this since it first came out, but I remember really liking it then.