They could have sex that was pleasurable for Alice, if Bob would focus on her needs and slow down/give her more foreplay/use more lube/etc...and if Alice would communicate that information to Bob.
Alice experiences pain during sex, due to hormonal changes, trauma, hang-ups about her sexuality or her relationship with Bob, health problems, structural issues, medications, or any combination thereof.
Alice enjoys sex as much as Bob does, and sees it as something to be enjoyed for its own sake, rather than as a means to an end.
Alice's libido exceeds Bob's stamina.
Gender Inverted: Bob endures rather than enjoys sex with Alice.
Alice is having sex with Bob, and seems to be enjoying it.
Alice and Bob's religiondoes say that they are allowed to enjoy the pleasurable aspects of sex within their marriage.
Alice and Bob are trying to make a baby, but they seem to enjoy doing so.
Bob is thinking about something not related to sex, so as to keep himself from finishing before Alice is satisfied.
She faked the orgasm so Bob would be satisfied.
But it also says they are not to use any sort of birth control (save for the rhythm-method or fertility awareness), because the act is first-and-foremost meant to bring new life into the world.
Zig Zagged: Alice starts off thinking she'll like it, but the experience ends up not being as impressive as she'd hoped. When Bob wants a second round she begrudgingly agrees, only for it to be more fun this time, up until something awkward or unpleasant happens.