I know that MSM folk are, because studies have confirmed it is a fact and I have seen it acknowledged on gay sites.
It is the reason that they are at such high risk.

From my linked page,

"Recent results from the Gay Men's
Sex Survey (GMSS) conducted
by Sigma Research suggests the
correlation between high numbers of
sexual partners and infection with STIs
including HIV – 'Numbers of sexual
partners was strongly associated to
HIV testing history, HIV diagnosis
and diagnosis of other STIs'. 3
For
example, 20.9% of those with 30 more
partners in the last year were HIV
positive (i.e had received a positive
diagnosis) compared with 6.6% of those
with between two and four partners in
the last year. Gonorrhoea was reported
in the last year for 16.1% of HIV positive
men with 30 or more partners and 6.6%
of men not tested positive with 30 or
more partners, compared with 4.9%
and 1.2% respectively for those with ..."