Re: luka romanov
1. Oooh. Um. Assuming "celibacy" means "no sexual activity with other human beings ever", I'd say the second one. Even if it gets a little boring, he'd still get that physical and emotional connection with someone else. He could live with that if he had to. If he really, really had to.
2. I think his relationship with his brother would be worlds different. I think they'd be more alike, they might even get along better, and they wouldn't be the source of each other's insecurities (and if they were, it would be different).
I think that, without his death looming over him, Luka would be less adventurous -- but, also, would have gotten so much more done with the last ten or so years. Instead of building organs, he would specialize in cybernetic prosthetics. He would have traveled more, slept with more people, gone on more adventures. He wouldn't have Extremis, and as a result would be much more outgoing and social than he is now, because the robot brain really took a toll on his personality and his desire to build very many close relationships.
He wouldn't be a superhero, for sure. He would spend more time on charity and social organizations, and he'd be a much bigger name in LGBT and sexual education circles. Without his health issues and the necessity of saving his own life like he did, he would be roughly a hundred times more active in anti-rape/sexual assault campaigns. He'd be significantly more political. He may have even gone public with his own experience with sexual assault; part of the reason he kept it so close to his chest when it initially happened was that he was petrified of wasting his life away, especially as a teenager, and felt that it would kick up too much fuss and eventually paint him as "that famous guy who accused lots of people of rape and then died like ten minutes later". He didn't want it to be part of his legacy, and now it's all so buried that he doesn't want to dig it up and properly deal with it. If he felt like he had an entire lifetime in front of him instead of less than fifteen years, he may have said something about it at the time.