There is stinks:
v. (intransitive, informal) To be greatly inferior; to perform badly.
This is comparable to the definition of sucks:
v. (intransitive, slang) To be inferior or objectionable: a general term of disparagement, sometimes used with at to indicate a particular area of deficiency.
Also, blows:
v. (intransitive, slang) To be very undesirable (see also suck).
Instead of saying "this sucks", one could say "shucks!" or "nuts!" This would depend on how much something "sucks", of course.
shucks: Interjection
(colloquial) Exclamatory response to a minor disappointment.
nuts: Interjection
Indicates annoyance, anger, or disappointment.
To describe the sex act, you could say fellatio.
Finally, you can opt for sarcasm: "that's interesting" instead of "that sucks". This relies on delivery and how well your audience knows you, otherwise your words may be taken at face value.