Buying a house before marriage?

Is it really that big of a deal if my SO and I aren't married yet but looking at houses?  We've been together for 3 almost 4 years and have a beautiful son and are recently expecting again so we're working with realtors to find a home for our expanding family.  
We want to get married, it's just life has thrown some curveballs.. Lol 
We've talked to a few realtors already and when they noticed were not married, they acted appalled that we'd even think about a house before marriage.  We're committed as committed could be and consider eachother as husband and wife even though we don't have the papers to say so. 
Also if you have any suggestions/tips about buying a house those are greatly accepted!! :))