Ok to look, but not ok to touch.
In our country (United States) nudism and seeing nude people is still, in many lifestyles, taboo, YES TABOO. Men look at breasts because they can. If you go to a beach, naturally there will be women that wear swimwear that tends to expose certain areas of their bodies. It is quite natural for men to be visually attracted to those images. A lot of it is just due to basic human instinct. It does not harm anyone unless he says things that make YOU uncomfortable. Look, don't speak and DON'T STARE!!!
Only if he doesn't ask and she keeps her shirt on. Mostly he SHOULD if he wants to...then don't say anything to him when he walks into a wall.
If they're his wife's breasts. It would depend on what your belief system is though. If you come from a biblical background than no it's not ok. There are many refrences in scripture that talk about lust and guarding your eyes. If you are from a secular background than I suppose you do what you feel is right. The belief that men simply can't control themselves is a lie and actually somewhat demeaning to men. We have spent to many generations teaching our little boys that they only need minimal self control to get through life and teaching our girls how to be little perfectionists.
Well....normally I don't think it is right for any men to be looking at women's' breasts. But it also depends what profession that your husband is in as well, if he is a doctor or something. And perhaps your husband is looking at other women's breasts to satisfy his sexual needs. Just show him your sexy side and have sex with him again and maybe he will concentrate on your boobs.