Seph is probably expecting more of Cloud than Cloud might be able to cope with.

That depends on if you're talking about in a sexual relationship not connected to the 'handler' issue or about starting it. What Sephiroth was hoping for in a sexual relationship with Cloud would probably be within Cloud's ability to give; attention focused on Sephiroth and a willingness to let Sephiroth be in control. But Sephiroth doesn't understand that Cloud might feel pressured into being with Sephiroth because that's what Sephiroth wants, no matter what Sephiroth might say to convince him that he won't retaliate if Cloud refuses.