The original question was regarding the morality of active homosexuality, not marriage. Trent says so what if the anatomy and physiology is not compatible (except in using the same public loos). Well here is a question, so what if a long lost 60 year-old brother meets his 69 year old post-menopausal sister and they fall in love. Is there any reason why they should not make out? Or can we say that it is just plain wrong? Or is it okay to have an affair and cover it up so no-one gets hurt?
The point being that some things should not be done because they are dishonoring to each other.
My impression is that society's interpretation of morality reflects their particular squeamishness about certain issues and changes according to time, technology and geography, which is presumably why you find morals are not the same in different countries and cultures at different times.
The UK prioritises individual freedom and happiness compared to many other countries. That's just the way the history of this country has shaped social norms. Society has become more morally pluralistic and multi-cultural rather than homogenous over time. Who knows - it may be that consenting brother and sister relationship or same sex sibling sexual relationships may be considered legal, acceptable (on par with other relationships?) in the future depending on medical advances in contraception and pre-implantation genetic diagnosis. It does not mean that a large percentage of the population would be interested in pursuing such relationships, but many in society may accept that it is ok that other people find them desirable - if medical risks can be overcome with technology. Of course there will be others who will continue to condemn such relationships since diversity in moral opinions is normal.