Steve is glad she comes when she does because he couldn't hold on a second longer, thrusting one last time and holding still as the orgasm hits him. It's not the same when he's wearing a condom, no, but he's not risking getting Peggy pregnant when they're not married and her work in the field is just as important (if not more so) than his. He's not going to compromise that.

"I love you. I love you so much," he manages, just holding himself on top of her and inside her for a moment longer. He wants to preserve these minutes forever.