I'm not familiar with the specific case, but:

Sin against nature or unnatural act is canon law speak for sodomy--a sex act that will never lead to reproduction. Note that in the medieval sources in which the foundations of canon (Church) law were hashed out, this is not just male-male homosexuality, although it frequently does refer to that.

Technically there were other sins against nature, but...right.

James Brundage, Law, Sex and Christian Society in Medieval Europe; Vern Bullough and John Boswell, eds., Sexual Practices and the Medieval Church