So, like every other obsessed webmaster/blogger, I go through my stats often and many times daily I get searches like "orgasm after IVF", "sex after IVF", "do orgasms after IVF prevent implantation", etc. So, a quick trip to Google scholar (this whole post has to be quick b/c I've got to run to get Matan from Taekwondo) gave me some information that's more scientific than my own personal experience (which I'm wise enough not to share here).
The theory was actually that intercourse may help implantation, despite the problems like uterine contractions & possibility of infection (due to the fact that the "cervical mucus barrier that prevents ascending infection is disrupted by passage of the embryo transfer catheter". - Aflatoonian et. al., 2009 - see the full article here). They found no significant difference between those who had intercourse within 12 hours of embryo transfer and those who did not (although the clinical pregnancy rate was higher in the study group than in the control group - 14.2% vs. 11.7%, it was not statistically significant).
A previous study by Tremellen et. al. (2000), cited by 50, showed a higher percentage of viable pregnancies in the intercourse group than in the control group. They reached the conclusion that "...Exposure to semen around the time of embryo transfer increases the likelihood of successful early embryo implantation and development." (See the abstract here.)
Based on this, I think it's pretty safe to say that it's OK to have sex - and even orgasms - after embryo transfer. It's not going to harm your chances of success. It might even help.