I have the same thought as Blondie. I don't think it really has to do with these angels wanting to experience sexual pleasure at all. It has to do with the ability to reproduce. Angels cannot yet they observed lower forms of life - animals - reproducing. The only other being they knew of who could bring living things into existence was God himself, who also brought them into existence. I can understand why they'd be jealous.
Add to that, because of the rebellion in Eden Satan and all the angels who sided with him received a death sentence with no hope of a resurrection. Yet, Adam and Eve sinned, but although they too received a death sentence they at least were allowed to procreate. In that sense, Adam and Eve would get to live on. In addition, as human society grew man began to rule over one another whereas Satan was punished for trying to rule over humans.
These angels figured out a way to get around all this. Materialize as "super" men, bully the human males, take their women, and create a hybrid race. Maybe take over rulership of the planet? But then God found a way to ruin their plan. By bringing the flood, the only way these offspring could survive was to dematerialize again. So essentially God killed the women and the hybrid children, but not these angels. However these angels weren't able to bring any women back with them.