I have become a fan of the baby monitor. Not for babies....lol.
We just got licensed as foster parents and due to the configuration of our house, it meant hubs and I moved downstairs, the 13 year old has our old bedroom and we have a bedroom with bunkbuds (those 2 share a jack and jill type bathroom).
We put a baby monitor in the hall upstairs right out off the bedrooms. we can't hear whipsering, but if anyone *talks* in a regular or excited voice we will hear it. Also if anyone needs help in the bathroom we would hear them if they call out.
We could also hear if goblins come in the house... unless they're the stealthy kind.
The baby monitor has done wonders for my peace of mind.
To answer your question.... no, I don't think it's irresponsible - especially not with kids your kids ages.'
And - we generally only close the door when we're having the monkey-love-time. However, one time my 13 year old rounded the corner to our bedroom in a full dead run of excitement to share good news about some celebrity or something-or-another and caught hubs (her step-father) with no shirt on. Now, she's seen him with no shirt on plenty of times, as we practically live at the pool all summer. But you'd have thought she ate poisoned poo by the look on her face. priceless. So, we were asked to close our door for *any acts of indiscretion* - Ah, gotta love a teenager with a large vocabulary.