(Sorry that it's super short, I'm gonna try and make them a little shorter, unless it's something really big, like Proposal, Wedding, Holding his Kid for the First Time, ect.)

Steve: Strangely, you and Steve became BF and GF over text messaging. It went something like this:
Y/N: Morning, darling
Steve: And you, Sweetheart.
Y/N: So I've been meaning to say something...
Steve: I'm sorry, but I have to go first. It's extremely important.
Y/N: All ears, darling
Steve: I've... I like you. A lot. Like, as more than a friend.
Y/N: That's what I was gonna tell you! XD
Steve: And I looked at your phone the other day... Saw my name was "Crush 😍😘 *heart eyes, kissy face*
Y/N: Yeah... 😉 *winking*
Steve: Well, I think you need to change it to Boyfriend.
Y/N: STEEEEEEEEEVE OH, STEVE! Yes!
Simple as that.

Tony: Tony did something small, surprisingly. On Valentine's Day, he simply gathered a group of friends to plan a small gathering and then at the end he handed you a box that had a heart necklace with your initials and his on the back. "Be mine?" he whispered. You nodded and had him put the necklace on, hugging him with your head on his shoulder. "I love you," you whispered. He looked shocked. "I... I love you too," the billionaire stuttered. You gave him a quick peck on the lips before turning back to your friends.

Bruce: He went all out for you, having a skywriter write you guys' names in a heart. You were on a lunch date during your break and you looked up to see it in the sky. "Will you be my girlfriend, Y/N?"
"Of course I will, Bruce." You smiled brightly and held his hand on the way back down to the lab.

Clint: Arrows. Always the arrows, with him. A note was tied to an arrow that went soaring by your face, nearly piercing your nose. You looked for Clint, but not finding him, resorted to looking at the note. In his terrible handwriting, he had written,
Y/N, I'm not really sure what to do with this. It's supposed to be a love letter, I guess. But really, I'll just get to the point, you know how impatient we both get. Basically, I love you, I won't leave you, I'll be the Ten to your Rose, yada yada. What I'm trying to ask, Peeta, is will you be my girlfriend?
Katniss/Clint
"Duh, you used TenRose. How could I say no, Clint?" you smiled. He popped out from out of nowhere and cupped your face, giving you a sweet, passion-filled kiss. "I hate TenRose," he muttered against your lips. "We'll be Everlark, then." You smiled before kissing him again.

Thor: He had no class, he simply just picked you up from work, kissed you and said, "This makes you my girlfriend now, no?"
"No, not really. But I will be anyways."
And that was that.

Loki: Being the God of Mischief, he thinks he tricked you into being his girlfriend, but really, you would have started dating anyways. "Oh, it's on. What happens when I win?" Loki asked.
"You're my slave for a month. And if you win...?"
"I have something in mind. But let's see who wins, first, no?"
"Tell me!" you gave him a stern look. "Uh, no. Now, I have three cards, all with a different face. One is the queen of hearts, the second is the ace of spades, and the third, King of diamonds..." and the game began.
"LIES! I won!" you pouted when the game had ended. "No. Oh, and because I won..." his lips passionately collided with yours, "we are now dating."
"What, we weren't before?" you joked.

Bucky: Bucky was extremely old fashioned when it came to dating. You'd been going out on dates for about four months, and he still hadn't confirmed what you guys really were. You had just gone on one and were standing on the porch of your house. "So..." you both said at the same time. You laughed, and gestured for him to speak first. "So, I'm not really sure how to put this. I was an extreme ladies' man back in the day, but now... Well, that's Tony. Anyways, do people even ask anymore? I'm going to, since I have no other way of doing it. So, Y/N," he pulled a box out of his pocket, opening it to reveal a necklace with a star charm (because of the star on his arm) that had the initials J.B.B. & (your initials) and continued, "will you be my girlfriend?"
"Bucky! Of course!" you gasped happily, throwing your arms around him. He attached the clasp around your neck after returning the hug. "So, what was it you wanted to talk to me about?" he asked, causing you to go into a laughing fit.