The opening of the baseball season brings joy to the romanticists and youngsters and other wholesome souls who love the summer game. But it also enthralls the degenerates to whom the national pastime means talking the Blue Jays at 9 to 5 and parlaying them onto the Yankees minus 2 1/2 runs.

"Baseball bettors," says Sneaky Pete, the bookmaker, "are a breed unto themselves. They're real sickies. In a way, they're a lot like horseplayers; they've got to be in action constantly.

"I've got one customer whose old lady won't even let him in the living rrom during baseball season. He spends the season in his attic with six radios and two television sets so he can follow all his bets."

Baseball gamblers can not only stay in action all day long, from the afternoon game at Wrigley Field through the night games on the West Coast, they can usually manage to survive throughout the long season because of the nature of betting on the sport. While football bettors have to deal with the subtleties of the point spread, baseball bettors merely have to pick the winner of a game.

For the uninitiated, baseball odds are quoted in terms of a $5 betting unit. If the Yankees are listed as a 7 1/2-8 1-2 favorite over the Red Sox, a man who likes the Yankees must risk $8.50 to win $5. A Red Sox bettor puts up $5 to win $7.50. The gap between 7 1/2 and 8 1/2 is what enables bookmakers to drive Cadillacs and grinds down bettors in the long run.

Still, baseball is a logical, analyzable game, and experts can handicap it well enough to surmount the bookies' edge. For guidance on betting this season I turned to Tom Boswell, the Post's baseball authority. Boswell is such a sickie that he refuses to bet on his beloved sport, feeling it would be a sacrilege. But he offered a few observations that can help those of us who couldn't even watch the seventh game of the World Series without a financial involvement.

Contrary to the popular view of it, baseball is a game in which the home-field advantage is becoming increasingly important. Only three of the 26 major-league teams had winning records on the road last season. The reason, perhaps, is that baseball players are less motivated than they used to be, and the home team is likely to be less indifferent. The same is true of regular-season pro basketball games, but while the home-court advantages is fully reflected in basketball odds, in baseball it is not.

Baseball has become an increasingly streaky sport in recent years, Boswell maintains. The team that loses two or three games in a row is very likely to lose four, five or six Players as well as teams are streaky, too. Everybody was watching Ron Guidry's incredible succession of victories last season, of course, and the odds were prohibitive when he pitched. But who noticed that Mike Flanagan was 30-6 from one June to another, offering bettors an opportunity to ride on a profitable bandwagon?

At the start of almost every season, a mediocre team will be playing exceptionally well. In 1978 it was Oakland. In 1977 it was the Cubs and the White Sox. Despite the inherent deficiencies of such clubs, they can usually manage to hold themselves together for 80 or 100 games, and betting against them can be a risky proposition. But when they collapse, they collapse totally.

Conversely, the cream will rise to the top during September. The latter stages of the pennant races are becoming much like the pro basketball playoffs, when the good teams start getting serious and putting together long winning streaks. The time to bet against the Yankees is now, not next fall.

Speaking academically, Boswell said, "I think you could make money on baseball taking home teams that are playing well, against teams that are slumping, especially if you've got the right kind of pitching matchups. Of course you'd have to wait for your spots. You couldn't bet 10 games a day."

That's what he thinks.