Hey there, straight adults: Atlantic Theater Company offers Let’s Love!, a trio of short, not-so-sweet comedies on heterosexual affairs. Oh, the rest of us can enjoy the program, too, since love’s emotional confusions are known to most people, one way or another. Neil Pepe, the company’s artistic director, stages the plays in a typically neat, well-acted Atlantic production that premiered on Wednesday. All that, plus an impish Nellie McKay has been engaged to stroll out and croon little tunes at a baby grand between the acts. There’s no intermission, so you’ll be in and out of the theater in 90 minutes.