Let's assume for a moment that Skull Knight hadn't gatecrashed the Eclipse. What do you think Femto had planned for Guts & Casca after the rape-scene? Do you think he was just going to let the Apostles finish them off? Do you think he would've wanted to kill Guts and/or Casca personally and perhaps even prolonged their agony rather than giving them a quick death?
There are, of course, theories that Femto would've raped Guts next, which can't necessarily be ruled out (Griffith/Femto was arguably bisexual and used sex as a means to an end, more than anything). I have my doubts about that, though. I think he knew that raping Casca was a far more traumatic way to torture Guts and exploit both their insecurities that he wouldn't need to resort to that, but who knows?
There are, of course, theories that Femto would've raped Guts next, which can't necessarily be ruled out (Griffith/Femto was arguably bisexual and used sex as a means to an end, more than anything). I have my doubts about that, though. I think he knew that raping Casca was a far more traumatic way to torture Guts and exploit both their insecurities that he wouldn't need to resort to that, but who knows?