By allowing boys and girls to choose who they will work with, it will enable students to work better together, as working relationships with people will better enable people to work together. For students who, for whatever reason(s), are not as popular (due to learning and/or developmental problems, etc.), it can be much harder, as less popular students are left out and are forced to work with whoever they're assigned to work with. This kind of a situation, where students are forcibly paired off is much more risky, however: Either the two students who are paired off together sort of by chance can and will get along okay, or get along very poorly. If on the other hand, a student prefers to work alone, s/he should be able to do that, as well.
Most of the time, students know who they'll work well with more than the teacher. Students choosing their partners and groupmates can draw them closer to each other and build strong friendships. People say that assigning groups prepares students to work together in the real world, but I say no.
Honestly, I have worked in group projects where being paired with a friend was the worst decision that could occur. What ended up happening every time was that we did not work efficiently because we just ended up screwing around, and in one example we did not agree thus conflict unnecessarily arose. It's better to just get the work done and achieve a higher grade than slack off and count on your friend. It's no coincidence that people suggest you don't have relationships with your co workers. Sure dating and friendships are two different things but the same effect/aftermath could happen, which would make both work (class in this case) and the chemistry between the people awkward. You can still hang out with your friend outside of class/the project but still get an A working with someone else. Not to mention, it's a great way to meet others and possibly make new friends. Although there have been projects where I have worked with strangers and shit hit the fan. However, I'd rather lose a stranger as a contact than a friend.