I don't think your being unreasonable but I don't think there's much you can do about it and in time it won't seem like such a big deal.
I'm the other way round IYSWIM, in that I'm a separated Dad and a few months after we split ex had a new BF. No problems there after the initial emotional blow, I behaved unreasonably to her - good for her. But when I was with DS for about 2 months he was saying new BFs name whenever he said Mummy (he was 18 months so didn't have a lot of words!) I assumed this meant that new BF was spending time with her and DS, though admittedly that is an assumption.
Anyway I think just out of natural parental curiosity I asked if I could meet him as would be nice to know who this person was if he was spending time with DS. I genuinely planned simply to say hello, no worries and maybe isn't DS great. I also thought it might just make things easier - on some pick ups he was leaving her house to avoid me before coming back again, which seemed a hassle that could have been avoided to me!
However ex didn't want me to meet him for various reasons - linked I think to the things that split us up so fair enough
In the end all I could do was let it go. She has subequently said he's no longer spending much time with DS anymore because they keep splitting up and getting back together so there we go and kudos to her for being a bit cautious on DS behalf in that situation.
Anyway I would echo what everyone else says - it is difficult and you are not being in any way unusual to feel that way. I am sure it is a natural feeling for any parent to be interested in anyone who consistently spends time with their children.
But you are Mummy, you always will be and though children like presents and fun what is most important to them is the support, understanding and love that parents provide.
Oh and don't forget what seems like a lovely, fun, family environment is never, ever like that 24 hours a day - the more people the bigger the chance of all the downsides of family life - disagreements / stress / etc.