NEW YORK — If you're flying this summer, be prepared to kiss your family goodbye at the gate. Even if they're on the same plane.

Airlines are reserving a growing number of window and aisle seats for passengers willing to pay extra. That's helping to boost revenue but also making it harder for friends and family members who don't pay this fee to sit next to each other. At the peak of the summer travel season, it might be nearly impossible.

Buying tickets two or more months in advance makes things a little easier. But passengers are increasingly finding that the only way to sit next to a spouse, child or friend is to shell out $25 or more, each way.

With base fares on the rise — the average roundtrip ticket this summer is forecast by Kayak.com to be $431, or 3 percent higher than last year — some families are reluctant to cough up more money.

"Who wants to fly like this?" says Khampha Bouaphanh, a photographer from Fort Worth, Texas. "It gets more ridiculous every year."

Bouaphanh balked at paying an extra $114 roundtrip in fees to reserve three adjacent seats for him, his wife and their four-year-old daughter on an upcoming trip to Disney World. "I'm hoping that when we can get to the counter, they can accommodate us for free," he says.

Airlines say their gate agents try to help family members without adjacent seats sit together, especially people flying with small children. Yet there is no guarantee things will work out.

Not everyone is complaining.

Frequent business travelers used to get stuck with middle seats even though their last-minute fares were two or three times higher than the average.

Now, airlines are setting aside more window and aisle seats for their most frequent fliers at no extra cost.