As @Pat noted, DDI indicates you can change gender. And the spell is a polymorph spell, which means that physical changes do take place. It would be difficult to disguise yourself via polymorph without taking the external sexual characteristics of the race into account. (Or removing them, in the case of lizardpeople.)
However, the fundamental question is one of the social contract between players and DM, because this is something that can completely and totally break immersion for some groups. While impersonating someone of the other sex is fine, (and I'm absolutely not getting into the silly prescriptivist debate between sex and gender), the question of bringing a child to bear as someone polymorphed into the other sex is one that is absolutely and completely under the DM's control.
Therefore, if a player tries this during your game, allowing them to look as if they're "Lady Such and Such" when they're a male rogue is fine. If they try to go further than that, ask them to table their plans for now and to chat with you after the game over coffee (or beverage of your choice.) Here, as in all contracts, the boundaries need to be drawn regarding the use of and the depiction of the act of sex in a game. I would be quite shocked if the player playing as a male character would want to carry a child within them for any non-trivial length of time, as that would basically stop the character adventuring. In short form, if they want to have sex, the most trivial thing to do is fade to black, and resume the story, as the biological consequences of sex are unlikely to be directly interesting to the plot.
However, in the right group, a story about the genetic nature of changelings could make for a very interesting philosophical exploration, if handled well. While this is a world-setting toggle "how do changelings have children?" the consequences are fascinating any way one sets the toggle. This topic absolutely must be unanimously agreed upon by the group beforehand, and is not a topic for a newbie DM nor a group that has people with extremely strong feelings on the topic.
My recommendation, if Changelings don't feature in your game except as a PC race is to assume that they have normal biological workings, are infertile except with people of their own species, and can mimic the external sexual characteristics (and appropriate mental responses) of any race their power covers.
If changelings do feature in your game, absolutely explore the question of how they reproduce, figure out the consequences, and then have it be part of their background or a clue in a plot.