Loki turns Steve into a Woman

[tmak94.livejournal.com]

I remember this one story where Loki turns Steve Rogers into a woman, and that the only way to turn him back is to marry (and do the nasty with) Tony Stark. The spell in particular was referred to as 'lock and key.'
I'm pretty sure that Steve becomes pregnant even after the transformation back, so it may also be considered mpreg. I cannot for the life of me find it again.