Re: ...Nikah vs. Engagement...
O-ho.. I am saying if he asks you for sex before marriage. Will you go ahead with marriage, even if the istikhaara had come out ok? Or will you say... nah! I don't think he is quite the right guy for me.
Ofcourse, if you already have nikah and are living together (or sleeping together) have as much sex as you want. Upto you.