You are not unreasonable to want a girl, I do believe that there's a certain biological yearning for a same sex child.
But it would be unreasonable to not look at it rationally and unpick why you want a girl and feel it would be different to a boy.
Firstly, I think a lot of the reason people get fixated on sex is gender stereotypes. This is particularly true with boys I think, as whilst it is seen as socially acceptable for a girl to wear blue or like rough and tumble, most adults who should know better seem to have a rather irrational fear of dressing boys in pink or if they like Frozen.
Studies do show that male role models are important both for girls and boys, so do be pleased your husband clearly provides that. There will be parts that you fill in that dad won't do for, but when you feel like the outcast parent you often overlook them.
Try to set up special time with your boys, separately, to nurture the relationship and don't feel disheartened if you don't get immediate responses from them. Children tend to fall into habits with caregivers, based on routine rather than preference etc.
Ultimately, bringing a child into the world is a joint decision and as you can't choose either a babies sex or personality, doing so just to get a girl would be unwise. Even if you did conceive a girl there's no guarantee she would fulfill your desire for a 'girl', as it is based on abstracts and there is no cardboard cut out 'girl' mum experience.
I recommend talking it through with yourself, husband, the dog etc and trying to work out where the need is coming from. If it's something like wanting to pass on unique experiences associated with being a 'girl', I'm talking navigating periods etc not liking ponies, then volunteering for girl guiding might help you meet those needs.
Finally don't forget that children are fickle with favouritism when it comes to parents and it will no doubt be your 'turn' soon.