Okay so… Tattoos are like my fave thing, and one of my favourite tropes is Tony having a Cap tattoo before they even meet. But I don’t think Steve would need any convincing, and I don’t think Tony would ever ask. He would probably be too scared that if he suggested it, Steve would end up regretting it. So here’s how I think it would go.
Steve would think of it one day when he kisses the tiny shield that Tony’s had on his hip ever since he was fifteen. Tony’s drifting off, eyes fluttering shut when Steve looks up at him, and it just hits him how much he loves this man, how much he wants to love him for forever.
So he decides right then that he’s going to get Tony’s arc reactor tattooed in the same place that Tony has Steve’s shield. He chooses the arc reactor because it represents Tony’s resilience, his determination, his brilliance. It’s proof that all he has he’s built himself, that if it all crumbled down around him he would roll up his sleeves and put it back together piece by piece.
Plus, it keeps Tony alive. That’s the most important thing in the world to him.
So Steve goes to get a tattoo for Tony’s birthday, and is completely dismayed when he finds out that the serum heals him too fast for it to work. He’s disappointed, but then he enlists Bruce to help him. It’s really important to him. He wants this so bad.
It takes months to get it figured out and Steve doesn’t manage to get it in time for Tony’s birthday, but he does get it in time for their anniversary.
They’ve just gotten home from dinner and Tony is sliding off his tie, kicking off his shoes and sitting down on the bed. He looks over at Steve who is standing by the door, hand still clenched around the knob. He’s thinking ‘this is is. It’s now or never’”.
Tony’s smiling at him as he steps forward purposefully, fond but a little confused. Steve is shrugging off his jacket as he moves, tossing it aside. Tony eyes it; it’s not like Steve to throw his clothes on the ground.
So when he’s finally standing in front of Tony – who is asking him what the hell is wrong, but Steve just shushes him – he starts unbuttoning his shirt. He’s standing in front of Tony, who has his knees spread so they’re bracketing Steve’s legs. Tony’s hand comes up to rest on the small of Steve’s back as he lets his shirt slither to the floor.
He rasps out a simple, nervous “I love you,” before hooking one thumb over the waistband of his pants and slowly drags the fabric over his hip, revealing the familiar spot of blue.
Tony’s eyes are immediately drawn down to his hand, and Steve can hear the hitch of his breath when he sees it, the was his eyes widen and his lips part in surprise.
He looks dazed, and Steve realizes he’s holding his breath as Tony stares. He says: “I thought it was about time I had a matching one.”
Tony looks up at the sound of his voice – his eyes are wet and the sight makes Steve’s throat close up a bit – and then his eyes flicker back down to the tattoo. He tentatively reaches out one hand and lightly brushes his fingers over it the marking; it’s tender still, but Tony’s hand is soothing and gentle.
Tony’s hands come up to brace themselves on Steve’s waist, fingertips digging into his flesh, his eyes never leaving the tattoo.
Steve almost stops breathing again when Tony leans in and brushes the softest kiss to the mark, his lips lingering for a long moment before he leans in and wraps his arms around Steve’s waist, burying his face into Steve’s stomach.
“Do you like it?” Steve asks, and Tony makes a choked noise, his arms tightening around Steve.
“I love it, and I love you.” His voice is muffled, but Steve can still hear how watery it is, the softness betraying the depth of the truth behind the words.
Steve starts running his fingers through Tony’s hair, and they stand like that for a long while, just soaking each other up, appreciating how much they love each other.
P.S. I actually already wrote a tattoo related fic <3