Because girls would be throwing themselves at him. And if he didn't have a GF he thought people might start questioning things. I mean if he is gay and wanted to keep closeted why not just say "I'm 100% focused on football right now and as a Mormon don't believe in sex before marriage".
But then he'd basically out himself as a 21 yr old virgin though but that didn't seem to hurt Tebow's draft stock or marketability.
Having a fake GF is an easy way to brush off any ladies trying to hook up with him, or any questions about his sexuality if he otherwise claimed he didn't have a GF but wasn't interested in having one either.