Sinful? For me to answer this, i'd have to...dispel some theories..
First off, what is a sin? A sin is basically an activity that is in direct opposing or disobedience to God's Law and Commandment.
So where in the bible was it stated that is wrong to use sexual toys?......waiting
Now, granted it isn't 'good' to use such toys as a single because it demeans the purposes of sex and sexual relationships plus it most likely leads to other methods of sexual gratification which ends in sexual immorality
But is it 'bad' as a couple to use such in their sexual activity? If the couples are okay with it, whats wrong? They aren't doing it anyone else besides themselves.
However, i would advised that if a couple would want to adventure into other ways of sexual gratification that those not involve a third person, they should sit down and talk about it. They could also contact a psychiatrist or psychologist (one of them is correct in this situation) in other to fully explore the emotional implications or side effects of this activity if any...
i rest my case..