Boys LOVE play kitchens (and even) as much as any girl. I've worked in preschools/day cares and it was a rare boy that didn't spend some time in that area. Oh, and my boys 5 and 2 love theirs as well.
Tell them to get a life. Role playing and imaginary playing is good . It's not going to make them gay to play with a kitchen, have a baby doll or even take dance classes. My son loves his, cooks me meals, pretends to do dishes. We got one of the little tikes ones that makes sounds when they are cooking. The only men that are concerned about their boys playing with kitchens are the ones that are not completely secure of themselves.
My first thought was- are they serious!!!!
I believe (and have been in classes where we have been told (uwm- sociology and psychology!)) that it is for all boys to play with kitchens, babies, doll houses, clothes, purses- WHO CARES!! They need to learn to be gentle, there is no evidence that he will be gay if that is what they are worried about! AND he's having fun! Why are gender roles so important?! More importantly why are we forcing our children into the stereotypical gender roles. If my son is a fabulous chef who helps me around the house when he's a I will be thrilled!
More down to earth- it's completely normal- just about all day care centers have them along with dolls etc. and if your son likes it I think it is a great gift. My 20 mo old has played with it since I can remember and it is a favorite toy of the 6yr old I watch after . It will grow with him- you can spice it up if he gets bored with new pots or or gadgets.
I think it's a great idea to get him a kitchen set. I wouldn't hesitate to get one for my son, who is close to your son's age. In fact, what's wrong with encouraging a boy to play with kitchen items? It's 2007 - surely we're beyond thinking cooking is only for women??? :)