Is it a "phase"?

So my boyfriend and I met in highschool, grade 9, and actually didn't start dating until grade 11. We have now been together for 3 years this October. 
​I really love him, and would do anything for him! He makes me happy and just, he is almost perfect except for a few immaturity flaws, and laziness. 
​Lately, I have been thinking about "hooking up". Like I would never, ever cheat on him. I could not crush him like that, but I can help but think about what it would be like to just "hook up" with another guy or two. I don't want to dump my boyfriend I love him very much. 
​Am I just going through a phase? What should I do?