OK, no to a guy, I have never been hit on by a guy yet I have many gay friends.
I am a kind "soul" last Easter I was asked by a lady I knew if I would have sex with her as it had been 7 years. I spent the weekend at her place, the sex was not good. I have both agreed and declined in the past. I don't have hangups about it, as long as I don't dislike the lady and she is not in a relationship, I am pretty certain I would agree most times. Since my last relationship ended 14 years ago there have been a couple of occasions where I have met someone and we are not interested in each other but the lady wished for sex. I like the direct approach, I am useless at reading people.
However if it was a total stranger in the street, I would probably run, same reason I don't pick up hitch hikers, if a lady is broken down I ring the breakdown service.

no thank you.

Girl, yes,but i would use protection. Guy, sorry, but I'm straight, thanks for the compliment

Simon Cowell: It's a no from me.

WTF?

These words come to mind:

What's in it for me?

I'd want to talk for a bit to get to know them first.

No or yes

Happened to me twice, said absatively twice, had a great time twice, went our separate ways twice. The first occasion wound up as a first person piece in a local rag. These are as defined by the OP, straight-up fuck me please deals. Ribaldry and flirtatiousness are different; they tend to lead to a relationship.

Probably ask if they have any condoms.

Men: No Thanks
Women: No, Yes, Maybe it would depend

Unrestrained laughter.

I guess it would depend on how long I had before I had to wake up from the dream.

If it was a girl I'd say no. If it was a guy, maybe. You never know.

I would agree with others here that say it would depend on a lot of different factors. When I was young and sexual freedom was the way things were this wouldn't even be a question. It's happened to me many times but not in the last 50 years or so. But these days there's just so much to consider. Like, do I feel in the mood and does she help me get in the mood by the way she handles the situation. Is there protection available, make sure this is not some film flam operation. and just how feverish I'm getting. I don't think any rational adult could say absolutely NO, or absolutely YES without figuring in the variables.

Got a condom

I'd tell them
Thanks but I'm married, thanks anyway, but just for your information, you're coming on a bit strong, a bit quick.

I would ask him what he would want me to do, and if I thought I would enjoy him. He was being honest!

I'd be flattered and it would do wonders for my self-esteem, and has.

Straight up I tell them, I don't do children so they can fuck themselves.

"We haven't even discussed my rates or anything yet. What's your name?" lol

Decide if I like the idea and if it is give them a kiss, not going to commit to sex without even a sampler.

Depends on what she looks like, how old she is, and if she is disease free or not. Other than that, two consenting adults, why the fuck not, have at it!

Cash or credit card?

By "straight up" I am guessing you don't know them yet. I have had that happen many times and I have to say it has caught me off guard every time. Do you go "Hell Yeah!" and dive for their skirt? Cuz I haven't tried that.