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.

“The customers that are more loyal, who fly more often, we want to make sure they have the best travel experience,” says Eduardo Marcos, American Airline’s manager of merchandising strategy.

For everybody else, choosing seats on airline websites has become more of a guessing game.

To travelers who haven’t earned “elite” status in a frequent flier program, flights often appear full even though they are not. These casual travelers end up paying extra for an aisle or window seat believing they have no other option.

But as flights get closer many of the seats airlines had set aside for those willing to pay a premium do become available — at no extra cost.

“Airlines are holding these seats hostage,” says George Hobica, founder of travel site AirfareWatchdog. “The seat selection process isn’t as fair as it used to be.”

Airlines are searching for more ways to raise revenue to offset rising fuel costs. In the last five years, they have added fees for checked baggage, watching TV, skipping security lines and boarding early.

Now they are turning to seats.

Since last summer, American, Delta Air Lines, Frontier Airlines and United Airlines have increased the percentage of coach seats requiring an extra fee. Some — like those on Delta, JetBlue Airways and United — come with more legroom. Others, including those on American and US Airways, are just as cramped but are window and aisle seats near the front.

Allegiant Air and Spirit Airlines go one step further, charging extra for any advanced seat assignment. On Spirit, passengers who aren’t willing to pay the extra $5 to $15 per flight, are assigned a seat at check-in. The computer doesn’t make any effort to keep families together.

“It gets really difficult, unfortunately, because all you end up with is a lot of onesies and twosies,” says Barry Biffle, Spirit’s chief marketing officer. “If you want to sit together, we would highly encourage you to get seat assignments in advance.”

Delta just launched a discounted “Basic Economy” fare on certain routes where it competes with Spirit that doesn’t include advance seat assignments.

“Airlines have to be careful. They can only push this so far before they risk incurring the wrath of customers or the government,” says Henry Harteveldt, co-founder Atmosphere Research Group.

Summer brings passengers traveling in larger groups and fewer empty seats. Last July and August, a record 86.4 percent of seats were filled by paying customers. Planes will be “slightly fuller this year,” says John P. Heimlich, chief economist at the industry’s trade group, Airlines for America. Add in seats occupied by off-duty airline staff and passengers who redeemed frequent-flier miles, and on many flights there won’t be a spare seat.

On a July flight from Dallas to San Francisco on American, a recent search showed only 28 of 144 coach seats available for passengers unwilling to pay extra. Of those, 21 were middle seats. There were five spots where a couple could sit together; groups of three or more were out of luck.

It was dramatically different for elite frequent fliers. They could pick from 75 seats including nine rows with four or more seats together.

Another flight — New York to Los Angeles on Delta — offered its most loyal fliers almost twice as many seats for free: 111 versus 60.

Booking through sites such as Expedia, Orbitz and Travelocity can add complications. If somebody inadvertently selects an elite seat or one requiring a fee, airline reservation systems won’t hold a seat for him. Passengers should confirm selections with the airline.

For those unable to find two or more adjacent seats, new seat assignments can be snagged for free starting five days before departure as some elite fliers are upgraded to first class. Another block of seats is released 24 hours in advance when online check-in starts. Finally, gate agents can sometimes put families in seats set aside for disabled passengers or ask others to move.

If a young child is separated from his or her parent, “we just have to get it done” says Frontier spokeswoman Lindsey Carpenter. “Usually, people are pretty accommodating.”

If all else fails, see if nearby passengers are willing to switch. There might actually be some chivalry left on planes. If not, offer to buy them a drink.