Many aliens DO look very much like us, often with some variations, and often much bigger. The humanoid form has established itself in this universe, it is one of the (if not the THE) most universally practical, adaptable and versatile forms. These beings are of cause always physically/genetically adapted to their environment, therefore the rather significant variations. Saying that they look like us would be looking at it from the wrong perspective. We look like them. We also have to take into account that our DNA is very much consisting of many of these species, our "evolution" was not natural in any way. We are generically engineered beings.
Although I don't know if we are sexually compatible in THAT way, I know that these beings, if we came into contact very often passed some of their genes on to us, or better to a specific group of peole located in that area (which is one of the reasons why we have so many "races" of humans here on earth). I don't know how they did that, but since we share some of their DNA with a few of them, I think it would work just fine. I don't know about other ones which we did not interfere with us, and I don't know if they would even want that in the first place. So far as I am aware they have not much interest in that anymore.
