I'm told that when getting a breast lift, I can't have sex for a few weeks.

   Increases in blood pressure can increase the risk for postoperative bleeding.  Incision lines can be disrupted with activity or swelling.

Thank you for your questions. I would advise that you follow your surgeon's instructions regarding post operative care. The idea behind avoiding sexual activity is that you do not want to do anything in the immediate postop period to raise your heart rate to cause a bleeding problem. Then after that, you want to make sure you are not doing any motions or activity which may disrupt the healing of the surgery. In the end, you just do not want to risk something fleeting which can cause long term consequences. Best of luck!

can be detrimental if your tissues are not fully healed but the increases in blood pressure and heart rate are not extreme or beyond what you may experience when stressed (GOOGLE it) so does it really matter?  You have to decide if you're willing to accept the risks and can do as you choose... worst thing that will happen is a potential hematoma and may need surgery to resolve.  There is no manhandling of breasts for at least a month in my opinion.  And if you want the blessings of your surgeon, you should follow his/her instructions.

Most breast lift recovery periods are a couple of weeks.  Especially during the first week, it makes sense to take the time to recover and avoid strenuous exercise and activity.  This is to avoid disruption of the incision lines with stretching, pressure, etc. as well as increasing your heart rate and blood pressure.  That said, using common sense and listening to your body will tell you when you are ready to resume normal activity.  
Cheers,
Brad Remington, MD

A breast lift is an operation that is well suited to a rapid recovery post-operative protocol.  With a rapid recovery breast lift or rapid recovery breast reduction, which is essentially the same operation, you should be able to do most things within 24 hours.  I have added a link and a video for more information.
I hope that helps.
Best regards.