The link between sex and love is primarily social or psychological, not biological. However it could be argued that social and psychological factors also determine survival of the species, so in this light, the sex-love link could be seen as natural because it contributes to better chances of survival.
The sexual act is largely determined by biological ("natural") factors, but its many permutations have come to lie under strong social and cultural restrictions and norms. Thus, the appropriateness of human sex is no longer determined directly by ovulation cycles or the sex partner's apparent health, but by psychological, social and cultural mores: is the partner of the right age, educational level, socio economic status? Will he be loyal, fatherly, the breadwinner? Will the sex be fun, dangerous, casual or amoral? Will I feel ashamed, relieved, guilty or scared afterward? For most people, the sexual act is surrounded by issues that are hardly biological, but they have become so integrated into human thinking about sex that they are considered "natural" concerns. Does "natural" = biological?
One of these concerns that appears central to debates about sex is love. However, this concern is fairly recent - in the past, sex was not consistently linked with love. In the Middle Ages, women of nobility married for political and financial reasons, but were not expected to love their husbands. Offspring were crucial to ensure the continuance of bloodlines, but love was rare in marriages. Marital partners sought love, but not sex, outside the marriage. This was considered acceptable, although extramarital sex was strongly condemned. This dissociation of sex and love suggests that the link between them is more a time - and context-bound ideology, influenced by general ways of thinking and Zeigeist than by any link inherent to them.
On the other hand, a couple who love each other and have sex may be more likely to provide a protective environment for their children: the child would be taken care of by two adults instead of one, thereby increasing chances of survival and better development. This is a fairly reductionist way of seeing the issue, but it does provide some evidence for a natural link existing between sex and love: it would increase survival chances, is therefore likely to be favoured by social selection, and hence, "natural". The context of affection and love would also be expected to be more rewarding for the couple, so from an operant conditioning perspective, the couple would be more likely to have more sex in the future, leading to a greater probability of procreation. This, again, would support the natural link between sex and love goes hand in hand with the increased emphasis an individualism in Western Culture. In the past, before the industrialisation of society, people were socially bound to their family and community contexts. A person was identified by his/her kinship ties and often through subordination to members of the ruling classes. The person was not seen in terms of individuality, or individual preferences. Love therefore was irrelevant, and marriage was linked to economic survival. Children were begotten in order to have more hands running the farm, or to provide for the parents when these became too old to work. In this context, love was not an issue, and sex was functional.
With the industrialisation and development of urban life, people were no longer tied to the farms they were born on - their livelihood went with them, wherever they went. With this change came also a change in identity - dissociated from kinship or serfdom, people now had individual identities, and individual preferences and needs. The individual became free to seek personal fulfilment - one element of which is love. This eventually effected a tremendous social change, and the link between sex and love was forged. People became freer to choose whom they wanted to marry, although religious control was still exerted over sexuality. A new value emerged: a person should marry someone s/he loves. Thereby, the two ideas converged: sex only within marriage, marriage to one you love - sex with the one you love. This link therefore emerged from the social climate of the industrial era and has been carried on to our age - although there are some changes on the way.
The 1960's and 1970 brought about another social change in Western thinking. Sex was no longer linked solely to marriage, and even love was not necessary. What mattered was the experience, enjoying natural behaviour. This was greatly aided by improved contraception, which seemed to remove serious consequences of the sexual act. Some groups entirely severed the link between sex and love, others retained it to a degree, while other sections of society held on to the older way of seeing the matter. This plurality itself was in the spirit of individualism: people could live according to personal preferences.
From the 1980's. with the awareness of AIDS, love and sex were joined again more, because stable relationships reduce risk. All these time periods illustrate a changeability in the thinking over time, and the influence of social and cultural ideology on sexuality. The "ideal" of sex and love linked, then, is perhaps more a situational factor - one that may change if it no longer fulfils society's needs. Seen another way, that link is "natural", given the present dominant ideologies and ways of thinking. This link is a natural manifestation of present-day individualism. Taken down to a lower level, a person's upbringing will determine whether they perceive the link between love and sex as natural or not. Vast cultural differences exist even today, when groups outside the sphere of Western influence are taken into account.