Serotonin
Endorphins
Vasopressin
Norepinephrine
Phenylethylamine
Dopamine
Oxytocin
Estrogen
Testosterone
Those are the chemicals associated with desire, sex, and love. It's true that positive chemical reactions tend to take place between people that become sexually involved. Yes, humans are wired to continue the species. Procreation is rewarded by reactions in the body for evolutionary purposes.
However, Oxytocin needs to be released during orgasm for the process of becoming emotionally attached to begin. Increase the number of times the reaction occurs during sex, and the emotional bond becomes more intense. People don't initiate that chemical cycle with everyone, or in every single sexual interaction. Nor will both parties always do it at the same time, at the same percentage, with everyone they encounter. Much less stick around long enough for it to build to a level that's consciously recognizable.
Lacking romantic emotions during sex, has nothing to do with whether an individual is a sociopath or not. It's merely the nature of certain chemical reactions. What about couples that no longer love each other anymore, but continue to have sex. Or one person that's in love with someone, but it isn't reciprocated. Yet, they too have sex. Or one night stands, friends with benefits, or any other casual sexual encounter. This happens all the time, far more than interactions that lead to love.
Sensation doesn't equal emotion. Yet, emotions can bring up physiological sensations.
Sensation:
1 a: a mental process (as seeing, hearing, or smelling) resulting from the immediate external stimulation of a sense organ often as distinguished from a conscious awareness of the sensory process — compare perception b: awareness (as of heat or pain) due to stimulation of a sense organ c: a state of consciousness due to internal bodily changes <a sensation of hunger> d: an indefinite bodily feeling <a sensation of buoyancy>2: something (as a physical stimulus, sense-datum, or afterimage) that causes or is the object of sensation.
Emotion:
1 disturbance b: excitement2 a: the affective aspect of consciousness : feeling b: a state of feeling c: a conscious mental reaction (as anger or fear) subjectively experienced as strong feeling usually directed toward a specific object and typically accompanied by physiological and behavioral changes in the body.