Sexuality and sensuality
There is no clear borderline between the sexual and nonsexual enjoyment of touching, or grabbing someone else's body. For example, holding hands may or may not have a sexual connotation, depending on culture, situation and other factors. Although the most common form of heterosexual sexual intercourse is universally regarded as sexual contact, there is a wide range of other sexual behaviors that may or may not be socially, legally, or ethically considered as sexual relations. The distinction between the sexual and the nonsexual becomes relevant in judging appropriate behavior, in either a social setting or in the eyes of the law.

Some criteria that may be applied are:

the body parts involved (see also intimate parts) 
physical signs of sexual arousal 
subjective feeling 
Enjoying touching someone else's body implies enjoying one's own body also; the latter may also happen without another person; enjoying one's own body also may or may not be of a sexual nature. If it is, it is called autoeroticism.

The whole of one's sexual activities (including erotic dreams and waking sexual fantasies and daydreams) is called one's sex life.

Sexual relationships
Opinions and norms vary about whether an emotional bond of a certain intensity and durability should be a prerequisite for sex (see also below).

Like other primates, Homo sapiens use sexuality for reproduction and for maintenance of social bonds. It is widely believed that children are capable of feeling sexual pleasure, even if they are not yet able to engage in sexual intercourse with each other, and/or are not yet biologically able to reproduce. Yet, child sexuality has historically been severely limited in western societies; in the late 19th century, the hysteria surrounding so-called "self-abuse" (masturbation) among children reached its peak and fueled the adoption of circumcision (including female circumcision) in some cultures, including some African tribes. Female circumcision has also been observed in England performed by some of the immigrants from other cultures.

Many sexual activities can be engaged in by same sex or opposite sex partners. However some, most notably vaginal sexual intercourse, can only be engaged in by partners of opposite sexes, and others (such as tribadism and frication) can only be engaged in by partners of the same sex.

Types of partnership
Sexual partners can cover many types, including:

One-night stand 
Casual relationship (friends with benefits) 
Boyfriend or girlfriend 
Marriage or other committed long term relationship 
Illicit affairs 
Secondary or side relationships 
Prostitutes 
Any of these may be explicit, or hidden, deceptive or honest, and may include fidelity or not.

It is also possible to engage in sexual activity without a partner, or (in some cases) without a knowing partner:

Masturbation 
Sexual fantasizing 
Several paraphilias (transvestic fetishism, voyeurism, frotteurism, and so on) 

Sexual relationships
A key sexual behavior throughout the entire animal kingdom is the seeking of a sex partner. Humans are no exception to this rule. A sexual encounter can be the result of the sending signals indicating readiness for sex, and being receptive to reciprocal signals. Or, it might be the result of years of planning, through the use of cultural rituals such as courtship and marriage.

Common methods:

Arranged partnership - other adults (often parents) choose partners. In some cultures these are suggestions, in others, they carry the force of commands. 
Personal choice - a person chooses for themselves their own partner, according to their own wishes 
Status based roles - a high status person in some cultures may choose partners backed by the force of social custom, and low status persons have little or no choice or expectation of avoiding the same. (For example, some employer-employee liaisons, and droit de seigneur) 
Mutual trade - prostitution, or "both gain" type of arrangements. 
Additionally, the pool of available and acceptable candidates may be limited, to own town, own religion, similar status, tactically advantageous (eg to cement social bonds or make peace), and so on.

The fear of Rejection
The fear of rejection is common when trying to befriend a potential partner. If the participants are both sensitive to the other's signals, then they can detect quickly whether their sexual objectives are mutual. If they discover soon enough that their objectives are at odds with one another, then a conversation can end before either one loses face, and then each person can seek others with whom to communicate. On the other hand, if the communication results in an escalating sense of intimacy for both participants, then a degree of trust is established that mitigates the fear of rejection. Important Tips....and more...