Fans and critics have also noted significant homoerotic subtext in his roles as Chase Collins in The Covenant and Bucky Barnes in the Captain America trilogy. Bucky is one half of the most popular slash ship in the Marvel fandom, Steve Rogers/Bucky Barnes, which was tagged in more than 20,000 works on the fanfiction archive An Archive of Our Own as of June 2016. There is also a fair amount of slash fanfic written about Chris Beck, Stan's character in The Martian.
As far as I know, Stan has never publicly commented on his sexuality. However, in a 2011 article about his role on Kings, OUT Magazine reported that "It's just as well you get over Sebastian Stan's sexuality right off the bat. Yes, the actor [...] is, in fact, straight." He has been romantically linked to women, including Gossip Girl co-star Leighton Meester, Once Upon a Time co-star Jennifer Morrison, and most recently Spread co-star Margarita Levieva.