Does filing for greencard invalidate my valid B2 visa?

What are the dates of your I94? If you overstayed in this country, you should not leave US, because you might trigger 3/10 year inadmissibility bar. If you did not overstay your I94, and plan to travel to your country, get married and go back, it's ok. However, if you plan to travel to your country and come back to the US with the intent to marry, you will be committing immigration fraud.

Yes, it does. By law, every applicant for a non-immigrant visa (including B2) must prove to a U.S. consul that he has no intention to take residence in the U.S.; otherwise, the visa application has to be denied. Once you file an adjustment of status application, you have demonstrated the intent to immigrate into the U.S., and are no longer eligible for a non-immigrant visa. If you live the U.S., you will likely face an unpleasant situation at the airport on your return: an immigration officer will inform you that your visa is cancelled and you cannot enter the U.S. Eventually, after your attorney goes through the motions, USCIS will grant you (but can deny!) an entry on parole, but you will have several very tense hours, and might actually have to go back to your country and wait there for a consular processing. However, if your application packet will be prepared properly, you will receive an advance parole (i.e., a permission to travel abroad and re-enter the U.S. while your adjustment application is pending) in about 3 months from the date of filing.

Filing for a green card does not invalidate your visa but you may have a hard time to return using it once you are married. You should consult with an immigration attorney regarding the filing for a green card and how to be able to travel while the application is pending.

It is not that it invalidates it but it is that once you are an applicant for permanent resident, you indicate your intent to immigrate to the US which is inconsistent with the intent of the B-2 visa so you cannot travel on that visa. However, after you submit your petition for the adjustment of status, you will also apply for a travel document, known as advance parole, which will allow you to travel internationally while your green card application is pending. It typically takes about 90 days to receive that document. Once you have it, you can travel to your home country and return with that document to resume your green card process.

What you have is likely a B-2 visa valid for multiple short entries during the 2 year period and so, you must return to your country on or before the date noted on the I-94 White card by the immigration inspector or else, your B-2 visa becomes invalid. You can certainly apply to change your status to green card while here in the US if you marry your girlfriend. But once you file, your B-2 visa will be voided. You can file a Form I-131 Advance Parole to travel while your green card case is pending. It does not appear you know some of the important issues involved, and before making any decision that may cause you problems later, I urge you to speak with an immigration attorney.

Do not travel on your tourist visa when you apply for your green card. Just wait for the green card to be issued before traveling. If done right, the entire green card process should only take 3 months from start to finish. You can have issues if you leave the country on a tourist visa after marrying a US intention.

Yes, but you can file for advance parole to travel when you file the I-485.