Many women view masturbation as a completely different experience than partner sex. A woman considers jilling off... her own ability to give herself pleasure, an escape from her stressful day, a kind of meditation, a chance to explore and enjoy her own body without feeling obligated to look/act sexy, a chance to get herself off without having to worry about getting someone else off too.
My understanding is that most men see masturbation as a substitute for partner sex; since it's just not practical or possible for them to have sex every time they are horny. It's the most easily available and convenient way to scratch an itch.
So, with that HUGE difference of ideologies in mind...
Here's a list of some of the possible reasons that men feel threatened by vibrators/sex toys; perceived from my own experience and through talking to my clients (I am the senior product specialist of a sex toy retail site).....
1. He's insecure about if the vibe is better/more efficient at getting you off than he is.
2. He's jealous about you getting pleasure from something other than him; like you're cheating or that the vibes is an uninvited guest in your relationship.
3. He's jealous that it's socially acceptable for you to have a sex toy, but it's seen as weird if he has one (and thus is stuck with his hand).
4. He feels the toy will replace him in some way; that you might favor using it over having sex with him and eventually stop having sex with him.
5. He feels a general sense of competition with the toy.
6. He doesn't like the non-human element; it's a machine/object and that creeps him out.
7. He feels that your expectations of his performance in bed will go up as a result of using the vibe; and that he won't live up to them.
8. If it's even mildly realistic looking (or, in some cases, a penetrative vibrator of any kind), he's thinking about the fact it might be bigger than him.
9. If he has erectile dysfunction, he might be feeling especially insecure... like you've lost faith in his ability to get or maintain an erection for sex.
10. If you are one of the many women who just don't get off from regular intercourse, but get orgasms from vibrators designed for your body and pleasure... he feels like your orgasms are this secret you've been keeping from him, something your sharing with an object but not him.
Upset, competitive, angry, insecure; all these emotions are wrapped up in the general idea of what if means to feel "threatened".