I'm using Inteen, ACR, romancemod and noinstantlove (and many many others) and I'm not sure which hack causes that, but my Sims can propose committed relationship (adults too, because of Inteen), engagement and marriage while being best friends (LTR 50 or above), no romantic feelings necessary.
Yesterday I've married an elderly couple without even a single kiss before wedding.
But I admit they fell in love meanwhile because of autonomous flirting. There were whines of noinstantlove being too strict - it is not! Now with Seasons, falling in love should be easy only on Spring, and in other seasons almost impossible. I've never noticed any seasonal differences on that matter.
I'll check my hacks and figure out which one causes that desired effect.