if you get any pleasure out of the thought of the sex with them. if you get that fleeting "i wish that was mine" or 'what if' thought. if you allow them to be reduced to an idea of fornication to arouse certain feelings...then yeah...thats lust.
the same lust that can exist between a married couple.
the sort of lust we are all capable of. i know i am.
it is well known that during extreme sexual arousal people can find themselves fantasising about all sorts of things they'd find deeply repugnant in the normal run of things. the 'ends' in these states seem to justify the means. this is the problem with attachment to these orgasmic states. everything becomes subordinate to the pleasure. it is not an aware attentive state. orgasm in itself is not wrong. it is usually our first experience of the changing energies in our bodies. but like everything else we must explore it honestly. if we reduce it merely to the pleasure and chase it like a drug it becomes sinful.
the mind can arbitrarily attach a certain object or person to the opening opening that floodgate. hence homosexuality or peadophilia. is it really any worse to stick parts of your body in an animal than it is to do so with a human if the mind is engaged in the same way? after all we're all matter. i get a bit tired by heterosexual married couples thinking they are somehow beyond reproach. lust is lust. it is reducing anything to an object of desire. end of.
strip sex down to its bare bones(so to speak) and its people chasing a feeling,a sensation, desire and power. pretty much to the exclusion of all else. think about it. without all the stories we manifest in or minds its a pretty dull excerise.
excuse me if this is immodest. i have had 'bad' sex and minblowing sex but i know that it would take two really sorted individuals deeply communicting to have brilliant sex. and that takes time and commitment. and from what little i've observed about the broadened human soul...i don't think sex would be high on the agenda anyway.