Amazingly, a 4 year old has a pretty narrow view of the world, which tends to extend as far as their immediate family unit. When they start mixing with other kids, they find that other family units exist, and they can find that strange. Talking about it gives them the chance to learn that not all families are exactly the same as the one they grew up in.
Other family units in my kids' school involved 2 dads with an adopted child, a few single mums (including a domestic abuse victim and a widow), 1 single dad (widower) and several blended families. It's not brainwashing to talk about things that already exist within the school. SRE doesn't teach kids "you should have 2 dads" - it teaches kids "some people have 2 dads". If a child is confused and insecure by that, I would suggest that's on the parent.
It's also worth noting that a lot of the conversations don't just "come up" - they're forced on kids. Mother's day could have been a really awkward time for the widower dad, but it had already been discussed among the class, so his daughter wasn't the odd one out - she made him a really sweet "Daddy, you're the best mummy" card.
It's not a whole lesson weekly, mainly because reception age kids don't have a rigid lesson structure. It's taught little and often. You're also wildly over-estimating what reception age children are taught - for example, if a 4 year old tells you they did history at school today, it probably means they coloured in a picture of a Roman soldier.