Assalaamu alaikum,
(smile) This is a question that seems to be raised quite a lot, I've noticed. And I think it really comes down to cultural understandings. And linguistic nuances.
I remember some years ago visiting the Caucasus. And I was astonished to see a couple of male security guards, seemingly all over each other... out in public, at the airport! In Canada, if you see men just holding hands, this generally means they are homosexual. But this is not at all what it means in the Caucasus. It's just that in Caucasian cultures, it's normal for heterosexual men to show affection through physical contact. Affection, not sexuality, is what the touching is all about.
In Canada, it is normal and usual for married couples to hold each other's hands, or walk arm-in-arm (indeed, if you see an older couple like this, it is cosidered very beautiful, and testament to the committment and deep healthiness of their marriage!). It's not just a question of security, but of affection. Not sexuality, affection.
Of course, there are those (and I've seen this both here and the Caucasus) who express sexuality in public. It is not a social norm, but it happens. Within certain limits, in Canada, it is not illegal (that I know of), so people generally avert their eyes and try to ignore the embarassing behaviour. We too feel that such acts are best kept to private spaces. Outside of certain limits, it is actually illegal, and people can be prosecuted (indeed, there was a case in the news just recently about this).
If a husband and wife are holding hands in a caring way, in a society that understands this gesture as one of caring and not sexuality, I do not know of any ahadith that prohibit this. Indeed, I have read in the past that the Prophet (SAWS) displayed his care of his wives in public (unfortunately, I don't have the references; if anyone has knowledge of these, I'd be grateful for them to post them).
There are cultures that interpret any sort of public show even of affection as scandalous. (smile) And if I lived in those cultures, I would try to take care not to offend local sensitivities. But in Canadian culture, it is not only acceptable to show care, but it is actually subtly not-well-seen to be too distant towards your spouse; it signals to people that there is either unkindness in the relationship, or that the marriage is in trouble. So as I live here, I would try not to upset these particular local sensitivities.
May Allah, the Repeatedly Forgiving, have Mercy on us as we struggle to understand His Will.