DP wants to use my lovely new pilates mat (thick, spongy one), as a mattress when he goes camping at a festival this weekend.
He has form for taking without asking borrowing my belongings and/or lending them out to others without first seeking my permission, so my annoyance may be coloured by his past behaviours.
YANBU Your lovely new mat will absorb all the gross camping/unwashed body smells and will never be properly clean and useable for pilates ever again.
YABU It's no big deal and the mat can just be cleaned afterwards.