Why does he do it?"
A great deal has been written on this subject. No-one really knows. All social groups of men are affected. Perhaps it is genetic. After all, before birth all embryos start out as female, which explains why men also have nipples! Perhaps his mother wanted a girl, and kept telling him so. Maybe he was always brought up with girls, and was conditioned to behave like them. Perhaps satin and velvet are just more snuggly than denim and wool! Perhaps the grass always looks greener for the opposite gender, like in the advert below, where men have to pay more.
Notice ads rarely feature a man, but usually a rich, beautiful, fashionable and female model. Why? To give a subtle message that buying will make us feel more like we can live her fantasy lifestyle. Why assume men aren't influenced by all the ads aimed at women? They cartainly are. They may not rush out and buy that particular lipstick, but unconsciously some absorb the message - they want to be like that lovely young model too. Ads do work, but the result may be unexpected.
"Come on, I watch ads for men's razors but I don't use them... Well all right, maybe on my legs."
Probably none of the above are conclusive in themselves, it is a combination of factors. Think of the conflict that religion or race have caused, through ignorance. When we stop to think, we know that as with belief or skin colour, many shades are valid. We are none of us 100% male or female. Women are are usually about 10% male, and have hormones of the opposite sex both naturally, and from the food we eat. The opposite is also true. One example is that some women have more or stronger hairs on their bodies than others or always wear trousers, but are no less women. Some men eat more foods like tofu, which contains a lot of female hormones, or are really good at knitting, but are of course no less men. It is perfectly possible for a person to be born 60% male, 40% female inside (or even 50-50) without any implications for their sexual preference.
"Can I stop him?"
Well you could, but only temporarily. The chances are, you will only make it a big guilty secret. Then neither of you will be happy, and the issue will be unresolved. There is increasing medical evidence that some men really can have a female brain in a male body. A tiny area in the centre of the brain concerned with gender identity (not sex) has been proven to be small in females and those who want to look like them, but large in males. But he definitely can't stop himself, even if he promises to, so don't try to make him if you love him.
"How do I know if he is bi-sexual?"
Ask him! It is vital to understand there are different types of crossdressers or TVs (See Definitions). By far the majority are straight and find the thought of sex with a man unthinkable, but of course many feel as confused as you do, and a few are gay. One rule of thumb (but by no means conclusive) is to ask him does he want to look and behave like a soft, understanding normal woman, or a loud, tarty slag? If he wants to look like a prostitute, it is possible that he may want to behave like one. Equally it may well be only fantasy. Don't panic however, as it is also extremely likely that if he looks like a tart, it is just because he needs help dressing convincingly as a normal woman!
"What if he wants to actually change sex?"
If that is so, he is unlikely to be with you in the first place. In that rare case he is trans-sexual (TS), a a woman in the wrong body who needs to be reborn. A TS will be taking hormone treatment, desperately wishes he could have babies, and genuinely wants periods! A TV usually would only fleetingly consider having the gender re-assignment op. For him being a woman is a favourite weekend hobby (for some men it's cars or ball games). If he doesn't get a regular "fix" and some acceptance of this special role, he gets very frustrated. I am not making light of it, but it is a regular, very temporary gender switch.
A sex-change doesn't happen overnight, and there is no way a TV can be bothered with two years living full-time as a woman before the pain, considerable expense and risks of surgery, or the continuing rejection which may be met afterwards. Hormones such as Premarin (made from pregnant mare's urine!) are not prescribed or taken lightly even for a TS, due to side effects and increased risk of breast cancer and worse. The nearest he might come is fantasising about having real breasts; but he wouldn't actually do that, as the results would be too embarrassing when in male mode, and there is no need when such good false breasts are now available. Women young and old are increasingly fascinated by cosmetic surgery, but comparatively very few do it - and we wouldn't have to change our whole body, job, identity and life as a TS would.
"Isn't it a perversion? I can't help feeling disgusted."
It's true some TVs may seem slightly mad. After all, their female side is often trying to escape long-term severe repression, making for difficult social relations. But a perversion is far, far worse, i.e. when physical pain or kinky sex with children or even animals is involved. (If he is into that, I'd be worried - but not about crossdressing, by far the lesser issue!) Who cares if a minority of ill-informed people are 'disgusted'? It harms no-one. Most women fear the worst and are relieved, once they find out his underwear doesn't belong to another woman, and he isn't the local axe-murderer. Take it step by step. Don't spend too long analysing it, just enjoy it.
"He reads crossdressing magazines full of pornography!"
Yes, there are some pornographic magazines that court popularity amongst homosexual men by including "chicks with dicks", or crossdressing sluts in O.T.T. positions with banner headlines like "Real Hardcore Filth"! But magazines actually aimed at straight crossdressers feature provocative poses, but little nudity and no sex. There is plenty of vanity, to be sure, and to get a balance the magazines may feature a "sensational" story as well as the serious items. The most-read items though will be make-up and style tips, and reassurance from real women.
"But they have ads from creeps, and dirty phone lines!"
So do the newspapers. Adverts are usually from gay or "bi" men looking for a partner, which gives the totally misleading impression that the majority of crossdressers are all like that. A TV will rarely advertise for a woman in there, because he will either already have a woman partner, or else he knows very few women will be amongst the readership. Magazines however need the revenue from such ads and phone chatlines, which the publishers imagine will be more readily accepted than in mainstream publications.