The uterus or womb is a hollow, muscular organ where a fertilized egg (ovum) will implant itself and grow into a fetus.[page needed] The uterus lies in the pelvic cavity between the bladder and the bowel, and above the vagina. It is usually positioned in a 90-degree angle tilting forward, although in about 20% of women it tilts backwards. The uterus has three layers; the innermost layer is the endometrium, where the egg is implanted. During ovulation, this thickens for implantation. If implantation does not occur, it is sloughed off during menstruation. The cervix is the narrow end of the uterus. The broad part of the uterus is the fundus.[page needed]
Two animals coming together for the purposes of sexual reproduction is called mating. Most animals only mate when the female is at the point of estrus, which is the most fertile period of time in its reproductive cycle. In certain animals, sexual intercourse is not only used for reproduction, but has taken other functions as well. These animals include bonobos, dolphins, and chimpanzees which also have sexual intercourse even when the female is not in estrus, and to engage in sex acts with same-sex partners. In most instances, humans have sex primarily for pleasure. This behavior in the above mentioned animals is also presumed to be for pleasure, which in turn strengthens social bonds.
On the other hand, most religious liberals define sexuality-related labels in terms of sexual attraction and self-identification. They may also view same-sex activity as morally neutral and as legally acceptable as opposite-sex activity, unrelated to mental illness, genetically or environmentally caused (but not as the result of bad parenting), and fixed. They also tend to be more in favor of same-sex marriage.
The bryophytes, which include liverworts, hornworts and mosses, reproduce both sexually and vegetatively. They are small plants found growing in moist locations and like ferns, have motile sperm with flagella and need water to facilitate sexual reproduction. These plants start as a haploid spore that grows into the dominate form, which is a multicellular haploid body with leaf-like structures that photosynthesize. Haploid gametes are produced in antherida and archegonia by mitosis. The sperm released from the antherida respond to chemicals released by ripe archegonia and swim to them in a film of water and fertilize the egg cells thus producing a zygote. The zygote divides by mitotic division and grows into a sporophyte that is diploid. The multicellular diploid sporophyte produces structures called spore capsules, which are connected by seta to the archegonia. The spore capsules produce spores by meiosis, when ripe the capsules burst open and the spores are released. Bryophytes show considerable variation in their breeding structures and the above is a basic outline. Also in some species each plant is one sex while other species produce both sexes on the same plant.
The Roman Catholic Church teaches that sexuality is "noble and worthy" but that it must be used in accordance with natural law. For this reason, all sexual activity must occur in the context of a marriage between a man and a woman, and must not be divorced from the possibility of conception. Most forms of sex without the possibility of conception are considered intrinsically disordered and sinful, such as the use of contraceptives, masturbation, and homosexual acts.