This isn't a lie, but rather a poor excuse that doesn't really hold water, when one doesn't want to directly say "no".
The other subject wouldn't typically be expected to come up with a solution, and if s/he did, the nonscuse is typically followed up with another. If the other subject is intelligent, they will "get the drift" but if they are stupid, it may at nonscuse 5 or 6 become a requirement to directly inform that you dont wish to.
Sarah: "I can't, I have my kids to look after"
Mike : "Well I can come to yours?"
Sarah: "Not really, its a little messy with all the toys"
Mike: "That doesn't bother me Sarah"
Sarah: "And i'm expecting my mum to ring up at some point"
(Mike gets the picture she is giving nonscuse after nonscuse and gives up)
Sarah (horny @ work) : "James, come over to my desk and drop your pants)
James (impotent) : "errrm, I'd love to, but isn't it a little inappropiate?"
Sarah: "Nobody else is in the office for 3 hours, come it will be fun - i've seen you staring at my lady lumps all week"
James: "Well, yeah they are nice, but i'm kinda looking for someone on the internet and saving myself"
Sarah: "Dont be so correct, come on, i'll let you stick it in my mouth"
James: "well.... I don't have any condoms with me"
Sarah: "I'm on the pill James, lets skin on skin"
James: "I dont think you would like it, I've been to the toilet 2 times since my shower"
Sarah: "Thats cool James, dont you like the smell of sex?"
James: "Dont you get it?? I'm impotent"
Sarah: "Well whats with all the nonscuses - just fucking say so"