As you say... If you're married, have a girlfriend/boyfriend, and you still find masturbation/device better...
than something is wrong with your relationship.
I mostly agree with you here. Certainly a real vagina physically feels better on your penis than does your hand or any plastic device (never tried one of those, actually, but I can't imagine it's the same.)
But with the above comment, I'll take some issue. If you've been married to someone for, say, 20 years, you might just be interested in variety. It's why so many people cheat on their spouses. But this is a safer way to do it that is less harmful to the relationship. With masturbation, you can use porn and/or your boundless imagination. Sex is as much mental as it is physical, and you just might want to imagine having sex with someone else, or some other kind of sex act that your wife will not do (such as anal or irama). So some may prefer to masturbate, at times, rather than have sex the 1000th time with their wives. Not necessarily always, but sometimes. It doesn't mean there's something wrong. If there was something wrong in every relationship where people fantasize about sex with other people, then every relationship is in trouble.