Do you have a feeling your novio/a is sleeping with someone else? Here's a couple ways to confront them. The most common way is:
 
You're cheating on me!
¡Me estás poniendo los cuernos!
¡meh ehs-tAHs poh-nee-ehn-do lohs cwehr-nohs!
 
Cuernos means "horns." The literal translation of this is "You're putting the horns to me." This is an informal/colloquial way to say it but it's most common. A more formal way to say it is:
 
You're cheating on me!
¡Me estás engañando!
¡meh ehs-tAHs ehn-gahn-yahn-doh!
 
Which literally translates to "You're deceiving me" but if you're speaking to your Spanish girlfriend/boyfriend they know what you're talking about. If you want to confront them about why they cheated on you, you can say:
 
Why did you cheat on me?
¿Por qué me pusiste los cuernos?
¿pohr kEH meh poo-sees-teh lohs cwehr-nohs?
 
…or..
 
Why did you cheat on me?
¿Por qué me engañaste?
¿por kEH meh ehn-gahn-yahs-teh?
 
An unfaithful lover/spouse is infiel.
 
If you someone cheated you on a game or a transaction you can still say ¿Por qué me engañaste? The cheater would be called a tramposo.
 

Today I'm going to cover things related to being engaged in Spanish.
Notice the Spanish verb used is comprometerse.  You'll probably find this most common in Latin America. When in Spain swap out comprometerse for prometerse.
 
Here's some common expressions:
 
I'm engaged.
Estoy comprometido/a.
ehs-toy kohm-proh-meh-tee-doh/ah.
 
Who are you engaged to?
¿Con quien estás comprometido?
¿kohn kee-ehn ehs-tAHs koh-proh-meh-tee-doh?
 
We got engaged.
Nos comprometimos.
nohs cohm-proh-meh-tee-mohs.
 
We are engaged.
Estamos comprometidos.
ehs-tah-mohs kohm-proh-meh-tee-dohs.
 
They got engaged.
Se comprometieron.
seh kohm-proh-meh-tee-her-ohn.
 
They are engaged.
Están comprometidos.
ehs-tahn kohm-proh-meh-tee-dohs.
 
He (or she) got engaged.
Se comprometió.
seh kohm-proh-meh-tee-OH.
 
He (or she) is engaged.
Está comprometido/a.
ehs-tAH kohm-proh-meh-tee-doh/ah.
 
He proposed to her.
Se le declaró.
seh leh deh-klah-rOH.
 
"Fiancé" (man) and "fiancée" (woman) are indentical in Spanish and English. (They both stole from the French.)
 
Don't forget the ring:
 
engagement ring
anillo de compromiso
ah-nee-yoh deh kohm-proh-mee-soh

I added a new page to the Hook Up Spanish Phrase Guide about asking and responding about who you live with. Examples:

Do you live by yourself?
¿Vives solo/a?
¿vee-vehs soh-loh/ah?

Who do you live with?
¿Con quién vives?
¿kohn kee-EHn vee-vehs?

I live alone.
Vivo solo/a.
vee-voh soh-loh/ah.

I have a room mate.
Comparto piso.
kohm-pahr-toh pee-soh.

More Spanish phrases discussing who you live with over here.

Today I added some Spanish phrases to the Guide covering Travel details so you can talk to someone about how long you or them have been traveling, how long you or them are going to stay for, where staying and with who.

Who are you traveling with?
¿Con quién viajas?
¿kohn kee-EHn vee-ah-hahs?

Are you by yourself?
¿Estás solo/a?
¿eh-stAHs soh-loh/ah?

I'm on my own.
Vengo solo/a.
vehn-goh soh-loh/ah.

You don't need a magic bullet to get a conversation rolling. If you are traveling in a Spanish speaking country one of the most natural ways to open a conversation is to ask for directions or a recomendation. I added a bunch of these basic phrases to the Hook Up Spanish Phrase Guide.

A taste:

Recommendations:

What's there _____ around here?
¿Qué hay de _____ por acá?
¿kEH igh _____ pohr ah-kAH?

What do you recommend _____ around here?
¿Qué recomiendas _____ por aquí?
¿kEH reh-koh-mee-ehn-dahs _____ pohr ah-kEE?

more basic openers here

People starting out learning Spanish sometimes use excitado/a for the word "excited" since they are spelled almost the same. Excitado/a does mean "excited" but in a sexual way. If you're excited about something, like a vacation for example, you want to use emocionado/a instead. To describe something exciting you can use emocionante.

I'm excited. (horny)
Estoy excitado/a.
ehs-toy ehk-see-tah-doh/ah.

I'm excited. (enthusiastic, thrilled)
Estoy emocionado/a.
ehs-toy eh-moh-see-ohn-ah-doh/ah.

It was exciting.
Fue emocionante.
fweh eh-moh-see-ohn-ahn-teh.

Excitado/a is probably the most formal way to say "horny." Here's some others; caliente, cachondo/a (depending on the country can mean "horny" in a more humorous way) and arrecho/a (probably less universal than the others).

If you want to tell your lover that he/she is making you horny in Spanish remember they that they say "putting me horny" using the verb poner instead of "making me horny" using the verb hacer:

You are (really) making me horny.
Me estás poniendo (muy) caliente.
meh ehs-tAHs poh-nee-ehn-doh (mwee) kah-lee-ehn-teh.

Enjoy!

I added some new phrases to the Hook Up Spanish Phrase Guide covering basic sexual needs, wants and desires.

BTW some of the Spanish sex slang below varies from country to country, look up the English term in the Hook Up Spanish Dictionary for other slang.