On the Mass Effect forums, a poster by the name of HogOfDeath2.0 received a letter from the ESRB explaining the meaning behind the partial nudity in Mass Effect:
"...According to the publishers submission for Mass Effect, there are two non-interactive videos that depict a discreet love scene between the main character and his romantic love interests. Breast profiles can be seen during one of the videos. There is a scene in a bar where female characters wear thong underwear. This type of content is likely to have contributed to the rating and content descriptors ultimately assigned by the raters...
Entertainment Software Rating Board"
Community Coordinator Chris Priestly didn't deny the content of the email when he wrote "YOU have a responsibility to find out the information. HogOfDeath2.0 was also interested, sent off a letter/email and found out".