Sex in public, train stations or popular parks is crime. It is so because sex triggers strong opinion, most salient of which is morality. Even sexuality which is consistent with the witnesses own, may still lead to rage, disgust, discomfort or other negative emotion. It is a crime, public sex, because it may imprint upon a youths developing sexuality in a way which is unacceptable to the youth-in-questions family, friends and moral guardians. It is a a crime because it is unhygienic.
Above is an example of legal restriction of sexual expression. Other common restrictions of sexuality include sex as work, paedophilia and homosexuality. These are centres of intense emotion, debate and of arbitrary limitation, such as 18 being consistently appropriate for coming-of-age to adulthood. Public space should be mostly neutral for co-existence between chafing diversities. Sexuality of private individuals & communities are allowed a much greater freedom of sexuality & restriction.
Sexuality has legal boundaries of which transgression incurs penalty. The most basic of these boundaries are held to be universal, and additional boundaries are only relevant to the public space. Restriction of sexuality, should burden all legitimate forms of sexuality equally. Criminalising public sexual intercourse inhibits hetero- & homo- sexual public sex equally. A mild counter example may be a homosexual couple kissing in public who hear pedestrians cast insult, which a heterosexual couple do not. Verbal expression of internal emotional states is valid, however, this hypothetical example demonstrates a move against public equality as a moral ideal.
Value systems which draw their scheme from a belief in a Divine Being, or which otherwise de-value equality are a deeper discussion, which leads into what should be the universal restrictions of sexuality. Hopefully it is agreeable to you that public spaces should be neutral, and to that end you recognise equality as a great boon. Does proper management of public space mean orienting it to suggest movement towards the ideal of the manager – should the government restrict sexuality to suit its own purposes (more families –> more people –> more tax –> greater financial power)?