Not true. She doesn't get subtlety, so most attempts to woo her end with confusion, but she's asked for sex outright a couple times, and she's happily had sex in comic form at least once.
BaconMinion said:
Copulate? Since when could she remember long words like that?
She's not lacking vocabulary, she lacks common sense.
"Let's mate!" Also works, but she is definitely asking for sex, specifically copulation, and I felt "mate" was too ambiguous, given that the Japanese language loves to be ambiguous, and Okuu is certainly not being ambiguous right now.