A few things with straddles. First of all a "live" straddle is only valid in cash games and the straddle is usually double the big blind (So in $1/$2 NL, the straddle would be $4). And it's not always allowed so you need to make sure the cash table you're at allows them and under what circumstance. Sometimes everyone at the table must agree other times you just must announce it and pay your straddle prior to being dealt cards. You may never live straddle in a tournament. If you post double the BB in a tournament, it will simply be considered a blind minraise from UTG; you will not get to make the last action PF.
IYour essentially raising blind out of position to most of the table after the flop, and every knows you have a completely random hand. My advice is to not do it.
This isn't accurate. If the game allows a live straddle, then what you're doing from UTG is actually increasing the stakes for the hand pre-deal, assuming the role of a third blind (sort of like a Bigger Big Blind), and you get to act last that first round. You do this before your cards are dealt, so it's no different than any other random hand anyone can have. You don't automatically have bad cards because you straddled; you could wake up with AA and have the last action PF. Then once the cards are dealt, the person that was UTG+1 is now UTG and first to act.
Hmmm. And to accept the challenge of a straddle, the opponent(s) must also not seen their "hole" cards?
No. The new UTG player waits until after the cards are dealt, looks at his hole cards, then decides to call the straddle amount, raise, or fold.
EDIT:
Robert's Rules of Poker said:
12. In non-tournament games, one optional live straddle is allowed. The player who posts the straddle has last action for the first betting round and is allowed to raise. To straddle a player must be on the immediate left of the big blind, and must post an amount twice the size of the big blind.