Proposed resolution:

Add explicit.

namespace std {
template <class Key, class T, class Compare =
less<Key>,
class Allocator = allocator<pair<const Key, T> > >
class multimap {
public:
...
explicit multimap(const Allocator&);

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Addresses JP-7

Constructor accepting an allocator as a single parameter should be qualified as explicit.