I have no issue with same sex relationships at all. I would however be wondering two things:
a) Is it really necessary to have a whole day devoted to this, and is this the most effective way to do things? Is this literally a one-off, or is the school in the habit of devoting whole days to this issue or that issue? There is no end to the number of social issues (young carers, pet welfare, refugees, rainforests) that schools might potentially want to spend time on. If schools start packing out the curriculum with very large periods of time spent on this kind of thing, there is a danger that insufficient time will be spent on the various subjects that children are supposed to be studying.
I would have thought that at Y2 level, a better approach might be to just slot this kind of information in in a low-key way--why not share some books with children which have a same-sex couple in there, within the normal curriculum or story time etc., and then just deal with questions as they come up, making it clear that this is just a normal part of life and not a big deal?
b) At the risk of turning this into Yet Another Trans Thread, my experience is that discussing "same sex relationships" in isolation seems to be exception rather than the norm these days, and this kind of issue is usually discussed in the context of the whole LGBTXYZetcetc package. ESPECIALLY if an entire day is going to spent on this area. In which case I'd want to find out exactly what my child is going to be taught and told. If this is going to include the trans stuff, I'd be concerned. I don't want my daughter being fed stories about how "little Steve liked dolls and skipping, so it became obvious "he" is actually a "she" because only girls ever like those things."