It depends a lot on the content of the curriculum and how it is put. I want my son to learn first about relationships and then sex.
I want my son to learn the difference between love and lust. I really hope he does not have casual sex or one night stands. I want him to give real careful thought before having sex with someone. I want him to appreciate that condoms are not 100% and there is no such thing as safe sex, just safer sex.
I think a big reason for failure SRE in the UK is too much emphasis on sex and not enough on building good and long term loving relationships. By relationships, I mean friends, parents, brothers, sister, grandparents as well as sexual relationships.
Discussing how to treat people in life and make good relationships is worthwhile at any age. Provided that the presentation of material is done at an age appropiate rate I do not have a problem with this being done in reception.
Of course relationship education is the responsiblity of the parents, but it can be difficult for the best of parents. Many parents find it difficult to talk to their children and children find hard to approach their parents. My son's school assists with him learning numeracy and literacy, so why not personal education?