Scientists recognise the brain rather than the heart as the "source" of emotions such as love and it's the brain that controls the reproductive cycle in women and sperm production in men. But it is the parts of the brain involved in thinking such as the hypothalamus and its associated pituitary gland that regulate sexual cycles.
Sex is simple to explain if it is considered as a mechanical response to stimulation and there is a mountain of resources for KS3 and KS4 teaching of the topic. The feeling we call love is not so easy. Love may be traceable to the limbic system, a group of nerve cells linked to our sense of smell. Human behaviour is more driven by our responses to smell than we might care to admit. Our choice of sexual partner is much influenced by pheromones.
You could explore the development of perfumes (an extension of the pheromones of nature) by outlining first the use of the musk gland secretions of the civet cat and then showing how the technique of distillation can be used to extract scented oils from rose petals or even orange peel. This would link with KS3 schemes of work. Experiments on the sense of smell and the related sense of taste can be done at different levels. Ask pupils to identify the flavours of unlabelled varieties of crisps (salt 'n' vinegar, BBQ, chicken, etc) or those of Harry Potter's favourite sweets. More sophisticated work could be done at KS4 using substances classified as salt, sweet, bitter or sour to map the taste buds.