Of course this will vary by jurisdiction, but in the UK at least this could rise to the level of rape:
https://www.legislation.gov.uk/ukpga/2003/42/part/1/crossheading/rape
"1 Rape
(1)A person (A) commits an offence if—
(a)he intentionally penetrates the vagina, anus or mouth of another person (B) with his penis,
(b)B does not consent to the penetration, and
©A does not reasonably believe that B consents.
(2)Whether a belief is reasonable is to be determined having regard to all the circumstances, including any steps A has taken to ascertain whether B consents."
Obviously this would hinge on whether the masseur believed that the OP consented and, if so, whether or not that belief was reasonable. The comments in this thread demonstrate that at least some would not consider such a belief to be reasonable in this context. You presumably think otherwise.