1
general::
Also, straddle the fence. Be undecided, not committed, as in I don't know if I'll move there; I'm still on the fence, or He's straddling the fence about the merger. This picturesque expression, with its implication that one can jump to either side, at first was applied mainly to political commitments. [Early 1800s]
American Heritage Idioms
2
general::
Phrase(s): *on the fence (about something)
Fig. undecided about something. (*Typically: be ~; sit ~.) • Ann is on the fence about going to Mexico. • I wouldn’t be on the fence. I’d love to go.
McGrawhill's American Idioms And Phrasal Verbs