Blondie: Were those angels seeking sex (since they were neither male nor female in heaven) or power/legacy of having children, forming their own tribe and rulership? I believe it was not sex but the latter, building up their families and power and rulership on earth, something they could not do in heaven unable to have offspring.
But Gen 6:2 specifically says that they were enticed by the women's looks:
Gen 6:2: the sons of the true God began to notice that the daughters of men were beautiful. So they began taking as wives all whom they chose.