It wouldn’t matter to me at all. But admittedly, I may not be representative of men in general. Unfortunately, I think most guys would care. It probably depends on their level of jealousy/insecurity. If you mutually decided to see other people, then there really shouldn’t be a problem. That is in an ideal world. When it comes to sex, a lot of idealism gets thrown out for reasons that entirely escape me. How well do you know guy1? Is he the jealous type? If so, then you’re probably in trouble. Because I think you need to tell him. Remember what Benjamin Franklin said, ‘three may keep a secret if two are dead.’ Somewhere along the line, he will find out. I think if you want a real relationship to develop, you have to be honest. A relationship without honesty isn’t going to last anyway. good luck.