I't was definitely m-preg and I think they had twins, and maybe one other child. The most memorable thing I think is that they have a secret cave as a getaway—a nice cave, with a hot tub, somehow. I recall that Tony and Pepper visited them and Loki and Pepper became really good friends. I remember one part where Loki is pregnant and Pepper comes to visit and spends the day with Loki in bed, completely platonically, but it made Tony a bit jealous, and Thor has to reassure him that Loki and Pepper are just friends.
Odin and Frigga throw them a wedding in another realm, not Asgard or Earth. I think Steve gets drunk at the wedding (random). I'm going to feel like an idiot when it's found because I know I should remember it. I went through my bookmarks, but I think I'm mixing something up. Fingers crossed! Thanks in advance.
Actually two separate fics. First paragraph: "Two Princes / Learning How to Crawl" by Sans_Souci, chapter six. http://archiveofourown.org/works/458193?view_full_work=true
Last two paragraphs: "Gemini" series by themantlingdark http://archiveofourown.org/series/40333