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Variety is the spice of life!
On a profile, one pic to me, is not enough. It sometimes indicates laziness, or perhaps excessive self importance.
Let's face it, most guys are are on sites like this to meet other guys. Your profile is an advertisement. In many ways, there is little as complicated as a human being and so a single pic is unlikely to give much of an idea about the person!
Also from the physical aspect, some people hate or find repulsive what others love, so it helps to show more than just one aspect of yourself!
My pics tell quite a lot about me. Yes in many people's eyes, I am kinky with a wide spread of interests. One pic would just not give the right impression. A profile can only say so much about someone and a series of pics tell so much more!
Of course, as has been said by others, this is assuming that the pics are genuine and equally importantly, relatively recent.
I realise that not everyone will agree with my thoughts, but then it would be so boring if everyone were the same!!