The Merchant of Venice is basically the same thing as a modern day romantic comedy. It's got romantic elements, everybody gets married or close to it at the end, and there are comedic elements throughout. It can be dark at times. Shylock is one nasty guy, but a reader definitely would not call the play a drama.
Romance first. It is pretty romantic that Bassanio is willing to "risk it all" in order to woo Portia. No, his life isn't at stake, but it has got to be humbling for him to have to ask for money from his friend Antonio so that he can go through with his love search. Some people would argue that it's romantic that Jessica and Lorenzo are running away together to get married.
The play is not only about the romance of those characters though. They aren't just taking each other out to dinner with romantic candle light. Each couple is in love, and believes that the end goal of that love is marriage. Portia and Bassanio, Jessica and Lorenzo, Nerissa and Graziano. They are all in love, and they all want to be married. It's a standard Shakespeare motif to have the play end with a wedding or plans of a wedding soon. Hollywood loves to end romantic comedies this way, too. The characters romance each other, then fall in love, and get married.
As for the comedy; it's there. Lancelot's entire purpose to provide comic relief. He makes snide comments and bags on just about everybody else in the play. Apparently cross dressing was as funny in Shakespeare's time as it is today. Portia dresses like a man. Jessica dresses like a boy. Plot twist abound, too. The entire court case is one twist after another. First Antonio is winning, then Shylock, then Antonio, and it's treated with levity more often than not vs. a dark sinister court case tone.
The Merchant of Venice has it all: romance, love, and comedy.