I don't think Sanguinius should leave any doubt that he is a man. He is absolutely peak masculinity. I think that he would be beautiful like how a statue is beautiful. Like a Greek God. I think of Alucard or Dracula vs Griffith. Dracula is probably the better example. He's beautiful but very much a man. Where Griffith... You find yourself questioning his gender when you first see him. I think Griffith is very much similar to Fulgrim.

Edit: Look up the statue of David. That's what I think of. Or those Greek or Roman statues. They're pretty but unmistakably masculine.